8 2.1 How Do We Know

2.1.1 Grounds for Knowing

The terms “a priori” and “a posteriori” (both from Latin) are used in epistemology to differentiate between two ways of knowing – they are epistemological distinctions:

  • Propositions known a priori require no sensory experience of the world. Such propositions can be known independently of and prior to a specific experience . For example:
    • 2 + 4 =6
    • A circle is the set of all points in a plane that are at a given distance from a center point.
    • Blue is a color.
  • Propositions known a posteriori require sensory experience of the world. Such propositions can be known only after experience. For example:
    • There are six puppies in the litter.
    • This picture frame is square.
    • This circle is orange.

The necessary truth of an a priori statement can be deduced by reason alone, whereas the truth of an a posteriori statement is contingent, requiring experience or factual confirmation.

2.1.2 Grounds for Affirming Knowledge

The terms “analytic” and “synthetic” distinguish between two processes for affirming truth of propositions, or making judgment.

  • An analytic statement is true based on what its words mean; it is true by definition. The truth of analytic statements depends only on the meaning of the words in the statement. No experience of the world is required. For example:
    • A Billy goat is a male goat.
    • If Oprah Winfrey is single, she is not married.
  • synthetic statement requires experience of the world to be known. The truth of synthetic statements depends on the way the world actually is. For example:
    • This Billy goat has an unpleasant odor.
    • Oprah Winfrey is single.

While there is correspondence between these two sets of distinctions, there are subtle differences. It is important to remember that:

a priori” is not the same as “analytic”; the truth of an a priori statement involves knowing by means of reason, while the truth of an “analytic” statement comes from the meaning of the words.

a posteriori” is not the same as “synthetic”; while both require experience of the world, the truth of an a posteriori claim comes from the fact that it can be known through experience, and the truth of a synthetic claim is about literal verification of the way the world is.

A supplementary reading resource is available (bottom of page) on these sets of distinctions.

2.1.3 Main Epistemological Theories

Establishing a satisfactory theory of knowledge has been a pursuit of philosophers for millennia, since the time of the ancient Greeks. The endeavor has been and remains a dispute between proponents of rationalism and empiricism, the two main theories of knowledge. The essence of the conflict is about the relative importance and primacy of the a priori (our rational way of knowing) and the a posteriori (our experiential ways of knowing.)

Keep in mind that the while rationalists and empiricists have held strongly conflicting positions, there are theories of knowledge that take both reason and experience seriously. Still, holding certain beliefs can result in being labeled one, or the other.


For rationalists, the only dependable path to human knowledge is reason. The theories of rationalists may include notions such as: deductive/inferential reasoning, intuition (non-inferential immediate knowledge). and innate ideas. The latter is a rationalist proposal that holds that some ideas exist in the mind prior to and independently of experience. This position is sometimes referred to as “nativism.” Quite often, some combination of these elements may be involved in a particular rationalist theory. But with all rationalist theories, knowledge is acquired through a priori means, and reason prevails as the only dependable source of human knowledge, when compared to experience and empirical processes.

Intuitionism may be regarded as its own theory of knowledge. For the purpose and scope of this course, we will regard intuitionism as a variety of rationalism in which knowing relies on non-inferential mental faculties, not on sensory experience.


Empiricism is the view that all knowledge and ideas come from experience. Experience is essential for knowing matters of fact, and only a posteriori means can lead to genuine knowledge. To think something, we must first sense it. Empiricists reject the rationalist view that a priori processes can lead to knowledge, and they reject the notion that any ideas or concepts can be innate in the human mind. We begin life with a mind that is atabula rasa, (Latin for “a blank slate,”) according to some empiricists.

Rationalism and Empiricism: Some Comparisons

Do we begin life with a mind that is pre-loaded/equipped with innate ideas, or do we start with a mind that is a “blank slate,” acquiring knowledge only as we experience the world.?

Rationalists believe that at least some ideas (maybe all, depending on the philosopher) are inborn, that there is no reliance on outside input or experience to acquire either ideas or knowledge. The very foundation of knowledge exists in our minds. The pursuit of knowledge entails a priori reasoning; it involves deduction and can produce necessary truths.

Empiricists reject the possibility that any idea can be innate. Experience, the a posterioriworld of sensations, is the only source of knowledge. Knowledge is built from experience and involves inductive formulation of probable truths, based on experience of the world.

Given these typical differences, and before going on to look at specific philosophers, it is important to note that there are degrees to which a specific philosopher may conform to the rationalist or empiricist model. For example:

While a rationalist subscribes to the supremacy of reason, there may be possibility of rational involvement in sciences fostered by experience and/or inferior knowledge via experience.

An empiricist may not accept the mind as the primary, superior source of knowing, but may still embrace the mechanics of deductive reasoning in a subject such as mathematics

A supplementary video resource is available (bottom of page) on these models.

The next two sections of the Epistemology module will look more closely at rationalism and empiricism. You will meet some of the well-known proponents of each theory and learn about some ages-long disputes between rationalists and empiricists.

Supplemental Resources

a priori and a posteriori, analytic and synthetic.

Internet Encyclopedia of Philosophy (IEP) A Priori and A Posteriori Read from the beginning of this article, through Section 3.

Rationalism and Empiricism

The following short video provides a review of terms and concepts on this section and may help reinforce your understanding of the main differences between rationalism and empiricism: Rationalism Vs Empiricism


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