Immanuel Kant’s Critique of Pure Reason is a masterpiece in metaphysics designed to explore the possibility of synthetic a priori judgments. A synthetic judgment is one whose predicate is not contained in the subject; an a priori judgment is one whose truth can be known independently of experience. Kant therefore in effect questioned how it is that statements in which the idea of the subject does not involve the idea of the predicate can nevertheless be true and can also be known to be true without recourse to experience.
To make the question clearer, Kant offered examples of analytic and synthetic judgments. The statement that “All bodies are extended” is offered as an analytic judgment because it would be impossible to think of a body, that is, a physical object, that was not spread out in space; the statement “All bodies are heavy” is offered as a synthetic judgment, because Kant believed that it is possible to conceive of a body without supposing that it has weight.
The judgment that “All red apples are apples” is analytic because it would be impossible to conceive that something that was red and an apple could possibly not be an apple; the predicate is, in this case, included in the subject. The judgment “All apples are red,” however, is synthetic, because it is possible to think of an apple without supposing it to be red; in fact, some apples are green. Synthetic judgments can be false, but analytic judgments are never false.
A priori knowledge is knowledge “absolutely independent of all experience,” whereas a posteriori knowledge is empirical knowledge, that is, knowledge possible only through experience. Human beings can know a priori that all red apples are apples (and that they are red), but to know that a particular apple has a worm in it is something that can be known only a posteriori.
The question whether synthetic a priori judgments are possible concerns judgments that must be true—because they are a priori and can be known to be true without reference to experience—even though, being synthetic, their predicates are not conceived in thinking of their subjects. As an example of a synthetic a priori judgment Kant offers the statement “Everything that happens has its cause.” He argues that he can think of something happening without considering whether it has a cause; the judgment is, therefore, not analytic. Yet he supposes that it is necessarily the case that everything that happens has a cause, even though his experience is not sufficient to support that claim. The judgment must be a priori. How are such synthetic a priori judgments possible?
One difficulty arises at this point. Critics of Kant have argued that Kant’s examples are not satisfactory. The judgment that everything that happens has a cause is regarded either as being an analytic rather than a synthetic a priori judgment (every event being a cause relative to an immediately subsequent event, and an effect relative to an immediately preceding event) or as being a synthetic a posteriori rather than an a priori judgment (which leaves open the possibility that some events may be uncaused). A great many critics have maintained that Kant’s examples are bound to be unsatisfactory for the obvious reason that no synthetic a priori judgments are possible. The argument is that unless the predicate is involved in the subject, the truth of the judgment is a matter of fact, to be determined only by reference to experience.
Kant’s answer to the problem concerning the possibility of synthetic a priori judgments that pure reason—that is, the faculty of arriving at a priori knowledge—is possible because the human way of knowing...
(The entire section is 1515 words.)