3 Sources of Knowledge: Rationalism, Empiricism, and the Kantian Synthesis

K. S. Sangeetha

Chapter Learning Outcomes

Upon completion of this chapter, readers will be able to:

  1. Identify the main theories of the sources of knowledge, including rationalism, empiricism, and the Kantian synthesis.
  2. Employ each theory to reconstruct the origins of a given instance of knowledge.
  3. Differentiate the categories of knowledge that arise from the a priori/a posteriori, necessary/contingent, and analytic/synthetic distinctions.
  4. Evaluate the merits of each theory.


We all have many things going on in our minds, such as beliefs, desires, hopes, dreams, imaginary figures, knowledge, love, and hatred—to name a handful. Have you ever considered their source? How do they come to be part of the thinking process? How do they become ideas in our minds? Some philosophers attribute the source of our ideas to the senses, including the inward senses (such as emotions) and the five outward senses (sight, smell, hearing, taste, and touch). We might sense the world directly or indirectly through the thoughts of others. Some philosophers even claim that all our ideas must come from our senses. This claim holds that each of us is born with a mind that is like a tabula rasa (Latin for a “blank slate” or “blank tablet”) on which nothing is written and to which we add contents through experience as we become exposed to the world. Knowledge that is dependent on experience, or which arises after experience, is called a posteriori (Latin for “from the latter”). Since a posteriori knowledge is empirical (based on observation or experience), this view is called empiricism.

Opposed to empiricism is rationalism, the view that reason is the primary source of knowledge. Rationalists promote mathematical or logical knowledge as paradigm examples. Such knowledge can be grasped, they claim, through reason alone, without involving the senses directly. They argue that knowledge accessed through reasoning is eternal (i.e., it exists unchanged throughout the past, present, and future). For instance, two plus three remains five. Rationalists are impressed by the certainty and clarity of knowledge that reasoning provides, and they argue that this method should be applied to gaining knowledge of the world also. The evidence of the senses should be in conformity with the truths of reason, but it is not a prerequisite for the acquisition of these truths.

Knowledge that is independent of (or prior to) observation and experience is called a priori (Latin for “from the former”). Rationalists maintain that reason is the basis of a priori knowledge. But where do we ultimately get the ideas on which reason is based, if not from observation or experience? Rationalists tend to favor innatism, the belief that we are born with certain ideas already in our minds. That is, they are “innate” in us. Potential examples include mathematical or logical principles, moral sense, and the concept of God. While innatists claim that such ideas are present in us from birth, this does not guarantee our immediate awareness of their presence. Reason is the faculty that enables us to realize or access them. In what follows, innate ideas thus serve as the foundation of a model for rationalism. [1]

Rationalism’s Emphasis on A Priori Knowledge

Portrait of René Descartes by Frans Hals via Wikimedia Commons. This work is in the public domain.

French philosopher René Descartes (1596–1650) and German philosopher Gottfried Wilhelm Leibniz (1646–1716), two important rationalist thinkers, support the existence of innate ideas and their realization through reason. They argue that the truths revealed by such ideas are eternal, necessary, and universal.

For Descartes, there are different modes through which we acquire knowledge: some ideas are innate, some are externally sourced, and others are constructed by us. Descartes gives the example of the idea of God as innate in us, as well as the idea of one’s own existence ([1641] 1985, Third Meditation). According to Descartes, innate ideas like truths of geometry and laws of logic are known through reason independently of experience, because experience gives us only particular instances from which the mind discovers the universal ideas contained in them. Therefore, they are a priori. Descartes’s innate ideas have been compared to the stored information in a book. The ideas are in us, though not always present to the mind. Once we start reading the book, the contents reveal themselves to us, just as reasoning reveals our innate ideas to us. In other words, it is only through careful “reading” (thinking) that we come to understand which ideas are innate and which come to us from elsewhere.

Portrait of Gottfried Leibniz by Christoph Bernhard Francke via Wikimedia Commons. This work is in the public domain.

Leibniz  calls innate ideas “principles.” Like Descartes, Leibniz maintains that principles are accessed by reason. The universal nature of mathematical truths, for example, is not revealed by the senses. It is the faculty of reason that acquires universal truths from individual instances. Leibniz argues that a collection of instances based on the senses cannot lead us to necessary truths. At the same time, it is also clear that we can grasp many necessary truths, such as mathematics. Therefore, the mind is the source, which means these truths are there innately. However, innate ideas are not full-fledged thoughts for Leibniz: he holds that our minds are structured so that certain ideas or principles will occur to us once prompted by the senses, although they are not derived from the senses. Ideas and truths are innate in us initially as dispositions or tendencies rather than as actual conscious thoughts ([1705] 2017, Preface).

Opposing A Priori Knowledge by Rejecting Innate Ideas

The empiricist claim that all our knowledge comes from experience is in stark contrast to the concept of innate ideas. For empiricists, all knowledge is a posteriori, meaning acquired through or after experience. John Locke (1632–1704), a British empiricist philosopher, adopts two approaches to question innate ideas as the basis of a priori knowledge. Firstly, he shows that innate ideas are based on dubious claims; secondly, along with Scottish empiricist David Hume (1711–1776), Locke shows how empiricism is able to offer a better theory of knowledge through the a posteriori.

Portrait of John Locke by Godfrey Kneller via Wikimedia Commons. This work is in the public domain.

Locke starts by questioning the “universal nature” of innate ideas. He opposes the claim that innate ideas are present in all of us by noting that sufficiently young children, and adults without the requisite education, lack a concept of God or knowledge of logical or mathematical principles. Therefore, it is baseless to say that innate ideas are universal. It is through experience and observation that we acquire such ideas. That is, they are a posteriori ([1690] 2017, Book I).

Here Leibniz defends the innatist view from Locke’s objection by showing how children and those without the requisite education are capable of employing logical and mathematical principles in their everyday lives without understanding what they are or being able to articulate them in words ([1705] 2017, Book I). A child, to use an example of my own, knows without any confusion that she cannot be sitting in both parents’ laps at the same time. Similarly, those without formal mathematical training could still know that two adjacent triangular cornfields separated by a fence on their longest side can make a square cornfield by removing the fence that divides them. Evidently, as Leibniz argues, general principles of logic and mathematics are innate. But this does not mean that all innate ideas are universally held. It is possible that we all have innate ideas yet some of us are unaware of them.

Locke further argues, however, that there can be nothing in the mind of which it is unaware ([1690] 2017, Book II). Having innate ideas without being aware of them is not a viable position for Locke. An idea first has to be experienced or thought. How else could it be “in” the mind? On this point Leibniz disagrees with Locke: it is possible to have a plethora of ideas in our minds without being aware of them ([1705] 2017, Preface). For instance, suppose you absorb a “tune” playing in the marketplace without being consciously aware of it. The tune is not readily accessible or transparent to your mind, in that you cannot recall it; however, it may be recognizable upon hearing it again. So, it must have been “in” you somewhere in some sense. Similarly, an innate idea could be in your mind, without you yet being aware of it. We are born with the facility to realize innate ideas when favorable conditions obtain later in life, such as the ideas of beauty, justice, and mathematical truths.

Locke’s reply is that the realization of ideas or capacities in the right circumstances is applicable to all ideas—not just those which are purportedly innate ([1690] 2017, Book I). He challenges innatists to produce a criterion to distinguish innate from non-innate ideas. Leibniz responds with such a criterion: innate ideas are necessary (they must be true, cannot be false), whereas non-innate ideas are merely contingent (possibly true, possibly false). We can distinguish truths that are necessary (and therefore eternal on Leibniz’s view) from contingent truths dependent on varying matters of fact ([1705] 2017, Preface).

Empiricism’s Emphasis on A Posteriori Knowledge

Locke claims to show how the mind, which is like a tabula rasa at birth, acquires knowledge. For empiricists, experience alone furnishes our mind with simple ideas, which are the basic elements of knowledge. Once shown that all ideas can come from experience, it would be redundant to additionally posit innate ideas. So, does a posteriori knowledge lead us to reject a priori knowledge? Let us find out.

For Locke, knowledge based on experience is easy to understand. He asks us to suppose that we have innate ideas of colors and that we can also see colors with our eyes. In this case, since we don’t need to rely upon both, we go with our senses, because it is easier and simpler to understand knowledge derived from sense experience than from knowledge derived from some source of which we are unaware ([1690] 2017, Book I, Chapter ii, Para. 1). Here Locke applies the principle of Ockham’s razor, which suggests that as far as possible we should adopt simple explanations rather than complicated ones. [2] Simple explanations have the advantage of being less prone to error and more friendly to testing than complicated ones that do not add explanatory value.

The next question is whether a posteriori knowledge alone gives us adequate knowledge of the world. Let us take an instance of experiencing and thereby knowing a flower, such as a rose. As we experience the rose, its particular color, texture, and fragrance are the ideas through which we become aware of the object. But when we are not experiencing or sensing the rose, we can still think about it. We can also recognize it the next time we see the flower and retain the belief that it is sweet smelling, beautiful to look at, and soft to the touch. This shows that, in addition to sensing, the ability to form concepts about the objects we encounter is crucial for knowing the world. Experience also makes it possible for us to imagine what we have not directly experienced, such as a mermaid ([1690] 2017, Book III, Chapter iii, Para. 19). Such imaginings are made possible because we have directly experienced different parts of this imagined object separately. Conjoining these experiences in the mind in an ordered manner yields the imagined object ([1690] 2017, Book II, Chapter iii, Para. 5). Had we not experienced and thereby formed the concepts of a fish and a woman separately before, we would not be able to imagine a mermaid at present.

These considerations lead Locke to categorize all our sense experiences into simple and complex ideas. Simple ideas are basic and indivisible, such as the idea of red. Complex ideas are formed by the mind, either from more than one simple idea or from complex impressions ([1690] 2017, Book II, Chapters ii & xii). Complex ideas are divisible because they have parts. Examples include golden streets, an army, and the universe. My idea or concept of an object, whether simple or complex, can be ultimately traced back to its corresponding sense impressions.

Portrait of David Hume by Allan Ramsay via Wikimedia Commons. This work is in the public domain.

Hume, another important empiricist philosopher, writes of ideas as the “copies” of “impressions.” Impressions are “vivid” and “lively” as received directly from sense experience. Hume also allows inward impressions, including jealousy, indignation, and so on. Ideas are mental copies of inward or outward impressions, rendering them “faint” or “feeble” (try comparing a perceptual experience with recalling it from memory) ([1748] 2017, Sections 1 & 2). Hume argues that where there are no impressions, there can be no ideas. A blind man can have no notion of color, according to Hume. One cannot be born with ideas that are not derived from any impressions. So, there are no innate ideas for Hume. However, he agrees that our tendencies to avoid pain, or to seek many of our passions and desires, are innate. Here I would argue that even these tendencies are based on our sense impressions and the corresponding ideas we form from those impressions. The mental inclination to repeatedly seek pleasure or avoid pain comes to us only after the first incident of exposure to either sensation.

In contrast to Descartes, even the idea of God falls under the a posteriori for Hume. Since none of us has experienced God directly, Hume argues, there is no impression of God available to us from which to form the corresponding idea. In Hume’s view, our imagination forms this idea by lavishly extending our experience of the good qualities possessed by people around us ([1748] 2017, Sections 1 & 11). Given that even the idea of God can be derived from sense impressions, this lends further support to the empiricist claim that all our ideas are a posteriori. Therefore, according to Hume, the rationalist claims for the existence of innate ideas and a priori knowledge are mistaken.

The Inadequacy of the Tabula Rasa Theory

A weakness of the empiricist’s tabula rasa theory can be exposed if we can show that not all our ideas are derived from corresponding impressions. However, this would not mean we must return to the rationalist’s theory of innate ideas, as we shall see. The plan is to explore a third alternative.

The presence of general concepts in our minds shows there is not always a one-to-one relation between ideas and corresponding sense impressions. For example, we see different instances of the color blue around us, and from these instances we form a general concept of blue. This general concept is not copied from one particular impression of blue, nor even from a particular shade of blue. We also have abstract concepts (such as justice, kindness, and courage), which are not traceable to corresponding sense impressions. In such cases, we experience different acts or instances of justice, kindness, and courage. But if these abstract concepts are copied from their particular impressions, then only these instances—and not the concepts themselves—would be in our minds. It follows that concepts are formed or understood rather than copied. Similarly, relational concepts (such as “on”-ness, betweenness, sameness, and the like) are realized not by copying the impressions involved. In fact, there are no impressions at all corresponding to these relational concepts. We instead receive impressions of particulars standing in such relations—the cat sitting on the mat, the English Channel flowing between the United Kingdom and Europe, one minus one equaling zero, and so forth.

In sum, the formation of general, abstract, and relational concepts in our minds shows that an uninterrupted flow of impressions would not constitute all the ideas we have. Instead, it requires that from birth the mind is at least partially equipped with a structure or architecture that enables it to make sense of the raw impressions it receives and to form concepts where there is no one-to-one correspondence between impressions and ideas. It challenges the authenticity of a tabula rasa. This takes us to a stage where we need to figure out the indispensable third alternative, which can facilitate a more complete knowledge of the world. This necessitates a crossover between the a priori and the a posteriori, or a reconciliation of the two.

Percepts-Concepts Combination

The immediacy and direct nature of sensations, impressions, and perceptions make them certain.[3] Let us briefly unpack this idea. Consider whether we can ever be wrong about our sensations. It is commonly thought that while we can be wrong about what the world is like, we cannot be wrong about the fact that we are having particular sensations. Even if you are dreaming this very second, and there is no actual book before your eyes, you cannot deny that you are having certain sensations resembling a white page and black font in the shape of words. Therefore, our sensations are certain and we cannot doubt that they exist. However, it is possible that sometimes we are unsure how to characterize a particular sensation. For instance, you may see a flashy car and be unsure whether the color is metallic green or gray. So, you might get into confusion in describing your sensation, but that does not affect the certainty and indubitability of the sensation itself, of what is here and now for you.

German philosopher Immanuel Kant (1724–1804) argues that for our perceptions to make sense to us, they should be received into concepts that exist within our minds.These structures of understanding allow our minds to process the impressions that we experience. Unless the manifold raw sensations we receive from experience are classified into different categories of understanding, we cannot make sense of them.

Immanuel Kant by Johann Gottlieb Becker via Wikimedia Commons. This work is in the public domain.

For instance, the mind should have the ability to recognize whether two sensations are similar or different, to say the least. Without this ability, we cannot make sense of experience. Or consider that we also perceive that objects are in space and time, stand in cause-effect relations, and belong to the categories of unity-plurality, assertion-negation, particular-universal, and the like. Here again, we are incapable of understanding any experience that is not processed through these categories. Kant argues, therefore, that space, time, causation, quantity, quality, and the like are represented to us in innate structures or concepts that our minds are fitted with prior to experience.

According to Kant, these categories are transcendental in the sense that they bridge the gap between mind and world. They are hidden structures, bridges, or concepts that occupy the otherwise blank slate and mold our way of thinking and experiencing the world. Of course, these concepts also require inputs, or percepts (the immediate objects of awareness delivered directly to us in perceptual experience through the senses). As Kant’s view is famously expressed, “Percepts without concepts are blind and concepts without percepts are empty” ([1781] 1998, 209).

So far, we have seen through various stages that rationalism and empiricism are incomplete. Kant’s transcendental idealism (as his view is called) strikes a balance, reconciling the two accounts. He combines sensory input and inborn concepts into a unified account of how we understand the world. Before we conclude the chapter with the final step in Kant’s approach, let us return to Descartes and Hume once again, the two philosophers who most influenced Kant.

Synthetic A Priori Knowledge

Descartes thinks that reason alone can provide certainty to all human knowledge. Intuition and deduction are tools through which the faculty of reason operates. Intuition is the capacity to look inward and comprehend intellectual objects and basic truths. Being a geometrician, Descartes thinks that deduction (the type of reasoning whereby the truth of the conclusion is guaranteed by the truth of the premises) should be used for gaining knowledge of the world, starting with the input of “clear and distinct” ideas. [4] Since intuition is dissociated from the evidence of the senses, the truths it unfurls can be known a priori. The result is that substantial knowledge of the world can be acquired a priori ([1701] 1985).

According to Hume, there are two ways in which reasoning aims to gain knowledge of the world: through “relations of ideas” and through “matters of fact” ([1748] 2017, Section 4). Hume thinks that the method of deduction establishes relations between the ideas we have already acquired through experience (e.g., that a mother is a woman parent). These relations of ideas are the kind of truths that we find in logic and mathematics (for instance, the proposition that a circle is round). They are true by definition. Such truths are necessary or certain (their denials lead to contradiction). They are also known a priori, since they do not rely on how the world is. For this reason, relations of ideas and deduction do not yield substantive new knowledge of the world; the knowledge they impart is already understood by us (as the above examples show), even if our understanding is merely implicit within the premises of a deductive argument whose conclusion makes it explicit.

Matters of fact, for Hume, are based on observation and experience. Some of them are generalizations arrived at by induction from particular instances. Inductive truths are uncertain. They are at best probable, since they are dependent on how the world is. For instance, we have the experience of heat from fire so far; but we cannot be certain that this will be the case tomorrow also (maybe we will unexpectedly feel some other sensation like cold from fire). We expect that the future will resemble the past, but we cannot be certain about it. [5] Matters of fact provide us with a posteriori truths, which are contingently true (their denials can be conceived without contradiction). Since matters of fact are not true by definition, they add substantive new information to our existing knowledge, unlike relations of ideas ([1748] 2017, Section 4).

A rationalist initially, Kant was influenced by the division in knowledge made by Hume. Only a combination of reason and experience can give us adequate knowledge, according to Kant. He begins by providing an account of relations of ideas, which he terms analytic truths. In sentences that express analytic truths, the predicate term is already “contained” in, or is the meaning of, the subject term. For example, in the sentence, “a circle is round,” the predicate “round” is contained in the subject, “circle.” To take another standard example, in “a bachelor is an unmarried man,” the predicate “unmarried man” is the meaning of the subject term, “bachelor.” We cannot deny such truths without contradiction. They are necessarily true, which means that they’re true regardless of how the world is. Since we do no need to examine the world to tell whether they’re true, analytic truths are knowable a priori ([1781] 1998, 146, 157). [6]

Kant terms matters of fact synthetic truths: the predicate term is neither contained within nor is the meaning of the subject term. Synthetic truths are not true by definition. As such, it stands to reason that they are based on observation, and therefore must be a posteriori (although, as we will soon see, Kant argues that this is not the case for all synthetic truths). For instance, consider the proposition, “George the bachelor is a writer.” We have new information here about a particular person named “George” being a bachelor and writer, and experience is required to find this out. Since the opposites of synthetic truths are not contradictory, they are contingent ([1781] 1998, 147, 157). [7]

Kant maintains that only synthetic truths are capable of providing substantive new information about the world. That said, our sense experiences do not passively enter our minds, but do conform to our innate mental structures to facilitate knowledge. Since these structures work independently of experience, they are a priori. These innate a priori structures of our minds—our concepts—are actively engaged in making sense of our experiences ([1781] 1998). They do so by discriminating and organizing the information received in experience. But again, the ability to perform this activity presupposes that the world which furnishes both the information and our concepts is itself structured in a way that enables intelligibility. The particular ways in which the world must be structured—its space-time and cause-effect relations, for example—yield substantive truths about reality. These truths hold not merely because of the meanings of words or the logical forms of sentences. They are synthetic. And since we arrived at this result by way of a priori reflection, Kant argues that we possess “synthetic a priori” knowledge of the world—a previously unrecognized category of knowledge, now to be added to the standard categories of synthetic a posteriori and analytic a priori knowledge. (See Table 1 below for a summary of these categories.)

Table 1 – Categories of Knowledge
Combining the epistemological distinction (a priori vs. a posteriori) with the semantic/modal distinction (analytic/necessary vs. synthetic/contingent) yields four possible categories.
Epistemological Distinction: A Priori vs. A Posteriori Analytic/Necessary (Relations of Ideas) Synthetic/Contingent (Matters of Fact)
A Posteriori (Empirical) Category of knowledge: analytic a posteriori
Significance: Receives minimal attention (because it is not a primary source of contention in philosophical debates).
Examples: Mathematical truths (e.g., that the ratio of a circle’s circumference to its diameter is > 3) learned by physical measurement, a calculator, or testimony from a reliable source. (Although such truths are commonly considered analytic, Kant disagreed, classifying them as synthetic instead.)
Category of knowledge: synthetic a posteriori
Significance: Emphasized by empiricists.
Examples: Truths about the external world known immediately via the senses or scientific investigation.
A Priori (Rational) Category of knowledge: analytic a priori
Significance: Emphasized by rationalists.
Examples: The deliverance of pure logic; statements that are true by definition (known by grasping their meanings).
Category of knowledge: synthetic a priori
Significance: Controversial category posited by the Kantian synthesis. While truths in this category are contingent in the strict logical sense (their denial is not logically contradictory), Kant claimed for them a kind of metaphysical necessity (in that they hold universally and are eternal).
Kant’s candidates: Euclid’s axioms of geometry, basic features of space/time, metaphysical truths, and moral truths.

There remains the question of how our concepts discriminate and organize the information received from the senses. These goals are achieved through acts of synthesis. By “synthesis,” Kant means “the act of putting different representations [elements of cognition] together, and grasping what is manifold in them in one cognition” ([1781] 1998, 77).

Kant explains three types of synthesis: the process starts with “synthesis of apprehension in perception,” passes through “synthesis of reproduction in imagination,” and ends with “synthesis of recognition in a concept” ([1781] 1998, 228–34). For Kant, apprehension in perception involves locating an object in space and time. The synthesis of reproduction in imagination consists in connecting different elements in our minds to form an image. And synthesis of recognition in a concept requires memory of a past experience as well as recognizing its relation to present experience. By recognizing that the past and present experience both refer to the same object, we form a concept of it. To recognize something as a unified object under a concept is to attach meaning to percepts. This attachment of meaning is what Kant calls apperception (Guyer 1987).

Apperception is the point where the self and the world come together. For Kant, the possibility of apperception requires two kinds of unity. First, the various data received in experience must themselves represent a common subject, allowing the data to be combined and held together. Second, the data must be combined and held together by a unified self or what Kant calls a “unity of consciousness” or “unity of apperception.” Kant concludes that because of such unity, all of us are equally capable of making sense of the same public object in a uniform manner based on our individual, private experiences. That is, we are in an unspoken agreement regarding the mind-independent world in which we live, facilitated by our subjective experiences but regulated by the innate mental structures given to us by the world. In sum, Kant’s theory makes possible shared synthetic knowledge of objective reality. [8] In conclusion, by considering the debate between rationalists and empiricists culminating in Kant’s synthesis, this chapter has shed light on the issue of how we achieve substantive knowledge.

Box 1 – Kant’s Copernican Revolution in Epistemology

Astronomer Copernicus, or Conversations with God, painting ca. 1872 by Jan Matejko via Wikimedia Commons. This work is in the public domain.

In his Critique of Pure Reason, Kant sums up his epistemology by drawing an analogy to the Copernican Revolution (the shift in astronomy from a geocentric to a heliocentric model of the universe, named after Nicolaus Copernicus (1473–1543), the sixteenth-century Polish mathematician and astronomer):

Up to now it has been assumed that all our cognition must conform to the objects; but all attempts to find out something about them a priori through concepts that would extend our cognition have, on this presupposition, come to nothing. Hence let us once try whether we do not get farther with the problems of metaphysics by assuming that the objects must conform to our cognition, which would agree better with the requested possibility of an a priori cognition of them, which is to establish something about objects before they are given to us. This would be just like the first thoughts of Copernicus, who, when he did not make good progress in the explanation of the celestial motions if he assumed that the entire celestial host revolves around the observer, tried to see if he might not have greater success if he made the observer revolve and left the stars at rest. Now in metaphysics we can try in a similar way regarding the intuition of objects. If intuition has to conform to the constitution of the objects, then I do not see how we can know anything of them a priori; but if the object (as an object of the senses) conforms to the constitution of our faculty of intuition, then I can very well represent this possibility to myself. Yet because I cannot stop with these intuitions, if they are to become cognitions, but must refer them as representations to something as their object and determine this object through them, I can assume either that the concepts through which I bring about this determination also conform to the objects, and then I am once again in the same difficulty about how I could know anything about them a priori, or else I assume that the objects, or what is the same thing, the experience in which alone they can be cognized (as given objects) conforms to those concepts, in which case I immediately see an easier way out of the difficulty, since experience itself is a kind of cognition requiring the understanding, whose rule I have to presuppose in myself before any object is given to me, hence a priori, which rule is expressed in concepts a priori, to which all objects of experience must therefore necessarily conform, and with which they must agree. ([1781] 1998, B xvi–B xviii)

Questions for Reflection

  1. Given the assumption that the propositions below are known to be true, label each one as (i) analytic or synthetic, (ii) necessary or contingent, and (iii) a priori or a posteriori. If any are debatable, state your opinion and explain your reasons.
    1. All triangles have three sides.
    2. The figure drawn on the board is a triangle.
    3. If the figure drawn on the board is a triangle, the figure has three sides.
    4. It is not the case that [latex]1+2 = 5[/latex].
    5. Some birds can fly.
    6. All flying birds can fly.
    7. The sun will rise tomorrow.
    8. It is morally wrong to harm innocent people for personal gain.
    9. The average apple is larger than the average grape.
    10. “Mark Twain” and “Samuel Clemens” are different names for the same person.
    11. Mark Twain is Samuel Clemens.
    12. Water is H20.
    13. Water is more abundant on Earth than on other planets in our solar system.
    14. God either exists or does not exist.
  1. Choose your own example of a posteriori knowledge. Then write a mini-essay that carefully traces its origins in a plausible manner. Use as many of the terms in the word bank below as possible (but feel free to also use other terms that appear in the chapter, especially those in bold). For definitions, you may wish to consult the glossary.
Impression Simple/complex idea Percept Concept
Relations of ideas Matters of fact Tabula rasa Innate
A priori A posteriori Deduction Induction
  1. Explain, in your own words, the main arguments for and against innatism.
  1. Explain, in your own words, the main arguments for and against the tabula rasa theory.
  1. How is it possible to avoid both innatism and the tabula rasa? What is the third alternative?
  2. Many philosophers view synthetic a priori knowledge in a skeptical light. Why might this be a difficult category to make sense of? How did Kant explain and defend it? Summarize his view in your own words.
  3. Consider the claim that “There is no synthetic a priori knowledge.” If this claim were true, could it be analytic? If it were true, could it be known a posteriori? If the claim is true but cannot be analytic or a posteriori, would it have to be synthetic a priori? If so, is it possible to consistently hold this claim?
  4. Which do you find most plausible—rationalism, empiricism, or the Kantian synthesis? Summarize your main reasons for thinking so.

Further Reading

Blackburn, Simon. 1999. Truth: A Compelling Introduction to Philosophy. Cambridge: Cambridge University Press.

Critchley, Simon. 2001. Continental Philosophy: A Very Short Introduction. Oxford: Oxford University Press.

Ellis, Addison. 2014a. “Idealism Pt. 1: Berkeley’s Subjective Idealism.” In 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/07/07/berkeley/.

———. 2014b. “Idealism Pt. 2: Kant’s Transcendental Idealism.” In 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/08/11/idealism-pt-2-kants-transcendental-idealism.

Plato. (ca. 380 BCE) 2009. Meno. Translated by Benjamin Jowett. The Internet Classics Archive. http://classics.mit.edu/Plato/meno.html.

Russell, Bertrand. (1912) 2013. The Problems of Philosophy. Project Gutenberg. https://www.gutenberg.org/files/5827/5827-h/5827-h.htm .

Vernon, Kenneth Blake. 2014. “The Problem of Induction.” 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/05/26/the-problem-of-induction/.


Chomsky, Noam. 1975. Reflections on Language. New York: Random House.

Critchley, Simon. 2001. Continental Philosophy: A Very Short Introduction. Oxford: Oxford University Press.

Descartes, René. (1641) 1985. “Meditations on First Philosophy.” In The Philosophical Writings of Descartes, translated by John Cottingham, Robert Stoothoff, and Dugald Murdoch, 1–62. Volume 2. Cambridge: Cambridge University Press.

———. (1701) 1985. “Rules for the Direction of the Mind.” In The Philosophical Writings of Descartes, translated by John Cottingham, Robert Stoothoff, and Dugald Murdoch, 7–77. Volume 1. Cambridge: Cambridge University Press.

Ellis, Addison. 2014a. “Idealism Pt. 1: Berkeley’s Subjective Idealism.” In 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/07/07/berkeley/ .

———. 2014b. “Idealism Pt. 2: Kant’s Transcendental Idealism.” In 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/08/11/idealism-pt-2-kants-transcendental-idealism .

Guyer, Paul. 1987. Kant and the Claims of Knowledge. Cambridge: Cambridge University Press.

Hume, David. (1748) 2017. Enquiry Concerning Human Understanding. Edited by Jonathan Bennett. https://www.earlymoderntexts.com/assets/pdfs/hume1748.pdf .

Kant, Immanuel. (1781) 1998. Critique of Pure Reason. Translated by Paul Guyer and Allen Wood. New York: Cambridge University Press.

Leibniz, G. W. (1705) 2017. New Essays on Human Understanding. Edited by Jonathan Bennett. http://earlymoderntexts.com/authors/leibniz .

Locke, John. (1690) 2017. An Essay Concerning Human Understanding. Edited by Jonathan Bennett. https://www.earlymoderntexts.com/authors/locke .

Plato. (ca. 380 BCE) 2009. Meno. Translated by Benjamin Jowett. The Internet Classics Archive. http://classics.mit.edu/Plato/meno.html .

Quine, W. V. 1951. “Main Trends in Recent Philosophy: Two Dogmas of Empiricism.” Philosophical Review 60 (1): 20–43.

Vernon, Kenneth Blake. 2014. “The Problem of Induction.” In 1000-Word Philosophy: An Introductory Anthology. https://1000wordphilosophy.com/2014/05/26/the-problem-of-induction/ .

  1. Plato (ca. 428–347 BCE) can be treated as a predecessor of rationalism. In his dialogue Meno, Plato shows how innate ideas can be realized through reason ([ca. 380 BCE] 2009). In this dialogue, the main character Socrates (based on Plato’s real-life teacher), engages a slave boy in discussion. Through a series of questions and answers—an approach known as the Socratic Method—Socrates draws out of the boy a proof about squares. Plato argues that the boy did not learn anything new; rather, the questions merely prompted the boy to recollect knowledge he possessed prior to birth as an unembodied soul. Therefore, innate ideas are like forgotten memories; we might not be aware of them. This is Plato’s “doctrine of recollection” (as scholars have called it). In recent years, some linguists consider Noam Chomsky’s theory of language to be a modern scientific version of rationalism (though perhaps it is more accurately described as Kantian). Chomsky (1975) argues that human minds contain innate structures responsible for our capacities to process language. This is because our exposure to language itself is inadequate to account for our ability to speak and understand others. He claims that this innate ability is universal across all cultures, which reiterates the claim of the early innatists that universality is an indicator of innateness.
  2. See Chapter 2 of this volume by Todd R. Long for a discussion of the explanationist theory of epistemic justification, and Chapter 6 by Jonathan Lopez (especially Box 1) on probabilistic considerations in epistemology—both of which are closely related to Ockham’s razor.
  3. We find an endorsement of this view in the Anglo-Irish empiricist philosopher George Berkeley (1685–1753). His view of idealism is that only minds and their ideas (where sensations are counted as ideas) exist. We are only immediately aware of ideas, and so the physical world of objects does not exist independently of mind—only as a representation of a mind, finite or infinite. Therefore, Berkeley recommended “To be is to be perceived” (in Latin, “Esse est percipi”). However, we will not explore this view here, as we are focused on the more influential view that there is a mind-independent reality. For discussion of Berkeley, see Ellis (2014a).
  4. See Chapter 2 of this volume by Long for further discussion of Cartesian foundationalism.
  5. This is an aspect of “the problem of induction” that Hume is famous for. For an overview of the problem, see Vernon (2014).
  6. See Chapter 6 of this volume by Lopez for a discussion of analytic/necessary truths in relation to probability theory.
  7. Some philosophers, following Quine (1951), object to the analytic-synthetic distinction altogether.
  8. Kant’s theory and its consequences were interpreted differently by post-Kantian philosophers, leading to the famous analytic-continental divide in philosophy. On the continental side, some philosophers interpret Kant as saying that we cannot know things as they are in themselves (the noumena). We can know only how they appear to us (the phenomena), resulting in a form of external-world skepticism (the view that we lack knowledge of the external world), Husserl’s phenomenology (philosophical description of inner mental life free from the traditional distinction between it and external reality), or a constructivist view (the idea that we construct reality). For a brief overview of these issues, see Ellis (2014b). For a more thorough discussion, see Critchley (2001).


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Sources of Knowledge: Rationalism, Empiricism, and the Kantian Synthesis Copyright © 2021 by K. S. Sangeetha is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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