B2: Cognition is a priori when it has a source or origin which is independent of sense experience.
Insofar as the three modes of cognition are intuition, concept and judgment, we have the following types of a priori cognition.
Intuition has a priori forms, space and time. Their source is in human sensibility. Space and time themselves are sometimes referred to as a priori cognitions. The forms of sensible intuition "lie ready for sensations a priori in the mind, and so must allow of being considered apart from all sensation" (A20/B34).
Some concepts are a priori, having their source in the understanding. These pure concepts of understanding are called categories. "It is because it contains these concepts that it is called pure understanding; for by them alone can it understand anything in the maniforl of intuition, that is, think an object of intuition" (AA80/B106). The range of legitimate application of the categories includes, and is restricted to, all objects of human sensible intuition. The legitimate application of the categories to objects of sensible intuition is made through principles of the pure understanding. Attempts to extend their use to objects in general give rise to error.
Judgments are a priori when the link between their two elements (typically subject and predicate) has an origin independent of experience. That there is such a link can be discovered only through the use of the two criteria of a priori judgments: that they are necessary and strictly universal. Ultimately the a priori character of judgments is to be explained through the a priori status of their elements, a priori intuitions and a priori concepts.
Although all a priori cognitions have their origins in one of the human faculties, some have empirical elements mixed with the a priori elements. Pure cognitions contain nothing empirical, and hence are entirely a priori. The a priori judgment that every change has a cause contains the empirical concept of change, and hence is not pure. But the concept of cause is a pure concept of the understanding. Space and time are pure intuitions, but the judgment that bodies are in space is not pure, since the concept of body is empirical.
For a discussion of the necessity of a priori judgments, see instructor's comments on the first paper.