A51/B75: “The faculty . . . which enables us to think the object of sensible intuition is the understanding.”
Thinking is representing a thing through the use of general concepts. This is opposed to the function of sensibility, which represents individual things which are given in intuition.
Kant also describes the understanding as “the mind’s power of producing representations from itself,” which he calls spontaneity (A51/B75). The mind is able to produce pure concepts of understanding, which Kant calls categories. These are thoughts of an object in general. The central question of the Critique is why these self-originated concepts should apply to objects which are given to us through the separate faculty of sensibility.
A89/B122) "The categories of understanding . . . do not represent the conditions under which objects are given in intuition. Objects may, therefore, appear to us without their being under the necessity of being related to the functions of understanding; and understanding need not, therefore, contain their a priori conditions.
