Kant proposed that we could specify the form of any possible experience which he designated with the term "the metaphysic of nature" or "the metaphysic of experience." He contended that one could spell out and think out what the "form of any possible experience" must be. Consequently, this would lead to a body of doctrine that would tell us about the world, telling us what the nature of the world is.
In this sense Hume's categories of analytic and synthetic categories was inadequate according to Kant.
He advanced that because there is such a possibility the categories of analytic and synthetic propositions is inadequate and he proposed a third category that he divided into two sub categories, the form of sensibilities and the form of understanding.
By the designation "form of sensibilities" Kant intended to convey that one could spell out and work out the fundamental character of space and time. According to Kant's conclusions, these two categories are imposed on our experience by the nature of our sensibilities.
By such he was saying that space and time do not characterize things are they are in themselves but are inescapable modes of experience located with us. These dimensions do not exist independent of our experience. In other words, reality has no such categories as space and time. He thought he was bring in geometry and arithmetic as forms of sensibilities and were bodies of apriori truth.
By the designation "forms of understanding" or forms of thought, Kant endeavored to convey the idea that the fundamental principle of causal determinism and Newton's law of the conservation of energy. These were fundamental to forms of understanding.
All of this comes to us in terms of "forms" which are sense and mind dependent.
Part of Kant's mission was to investigate the nature of these internal forms. Remember, these forms are internal to the individual and not actually part of the world as it is. The implications of Kant's conclusions were tremendous.
It appears that Kant was intending to build a firm foundation for theological speculation about God or at the least metaphysical speculation about the cosmos. However, in the end Kant actually concedes that there cannot be any such foundations. This is unfortunately the case because all knowledge is limited to the experience and the apparatus through which we receive the experience and the issues, so to speak, of God and the soul for example do not meet this standard. They are not knowable to us.
