How might the human mind relate objects a priori, independently of the input of the senses? One possibility is that the mind employs a faculty which relates to objects directly or intuitively, bypassing the senses entirely. Another possibility is that the mind gets at its objects indirectly, through the use of general concepts, which in turn are not the products of sense experience. Kant held that both these traditional explanations of a priori access to objects are fatally flawed.
Philosophers from Plato onward have held that the human mind has direct insight into objects apart from their being given through the senses. Kant dismissed such claims to "intellectual intuition" of objects as mystical. His fundamental objection was that directly given objects are given nonetheless, and that no existing object can be given to human beings "as is." What is given in human intuition is always modified by the manner in which it is received. Only an "original" intuition, one which gives rise to the very existence of its object, would be able to represent its objects as they are "in themselves."
The difficulty with intellectual intuition is circumvented by the second approach to metaphysics, according to which the human mind is related to objects a priori through the use of general concepts. Kant held that although some of our general concept (e.g. that of a dog) are derived from sense-experience, others (e.g., that of a supreme being) are generated "spontaneously" by the human mind itself. Such concept-generation is therefore a priori, and the generated concepts are legitimate starting-points for metaphysical investigation. "The root and peculiarity of metaphysics," Kant stated, is "the occupation of reason merely with itself and the supposed knowledge of objects arising immediately from this brooding over its own concepts, without requiring experience or indeed being able to reach that knowledge through experience" (Prolegomena to any Future Metaphysics, Section 40).
Despite this solid beginning, metaphysics quickly takes a wrong turn, according to Kant. The a priori origin of metaphysical concepts suggests that they apply to objects which themselves are independent of sense-experience. In Kant's terminology, such concepts are used "transcendently" in a vain attempt to gain access to "hyper-physical" objects, such as God. But the applicability of pure concepts to non-sensible objects is a kind of illusion, the exposure to which spells an end to traditional metaphysics.
Kant's proposed an alternative metaphysics, which retains an a priori element, but confines it to objects of sense-experience. These objects are given to the human mental faculty of "sensibility." As mentioned above, Kant held that any object given to the human mind is subject to the means by which the mind receives it. Kant claimed that there are two "forms" of human intuition, space and time, and that all objects of sense-experience are therefore to be found in some region of space and at some period of time. Space and time themselves are not taken from experience, and hence are a priori forms. Kant regarded his "discovery" of a "sensible a priori" to be crucial to the explanation of mathematical knowledge. Objects of sense-experience conform to mathematical principles because the principles themselves are directly applicable to space and time.
Objects are given to the human mind in space and time, and they are thought through general concepts. Kant claimed to have discovered as systematic arrangement of twelve a priori concepts which he called "categories." The legitmate use of the categories is restricted to objects in space and time, i.e., the objects of sense-experience. Most prominent among the categories is that of cause and effect, whose principle is that every change in an object of sense-experience is determined by its prior state in accordance with a rule.
Attempting to extend the use of the categories beyond the field of sensible objects is what gives rise to the illusion that a priori concepts can be used transcendently. For example, the category of causality might be extended to encompass an uncaused cause. Such an object could not be met with in experience, which is subject to the principle that every change in an object is the outcome of its prior state and a rule governing its transition to the subsequent state. On the other hand, there is something profoundly unsatisfying to human reason in the postulation of an endless series of causes. So there is a conflict or "antinomy" of reason with itself.
Kant took such an conflict as symbolic of the futility of transcendent metaphysics. We must simply give up any pretense to knowledge of some ultimate or unconditioned object like an uncaused cause. If may be useful to assume that there is such a thing, but its existence can never be proved. On the other hand, its existence cannot be disproved, either. Kant held that the failure of transcendent metaphysics carries with it a highly desirable consequence. It allows for the possibility of human freedom, insofar as the human will is considered as a non-sensible object. This is of supreme importance for Kant's ethics.