He remained unmarried. His Critique of Pure Reason appeared in 1781, Prolegomena to any future Metaphysics in 1783, Fundamental Principles of the Metaphysic of Ethics in 1785, Metaphysical Rudiments of Natural Philosophy in 1786, the second edition of the Critique of Pure Reason in 1787, the Critique of Practical Reason in 1788, and the Critique of Judgement in 1790. In Kant's philosophy, knowledge is the outcome of two factors, the senses and the understanding.
Sensations are the starting point of knowledge.
Space and time are essential conditions of our sensuous perception, the forms under which our sensations are translated into consciousness. Therefore space and time are essential conditions of knowledge, although they exist only as forms of our consciousness. These forms, continuous and infinite, provide the possibility of unifying our individual perceptions, and the unification is effected by the understanding. This act of synthesis Kant analyses into twelve principles or 'categories', or laws of thought. The categories are to the understanding very much what time and space are to the consciousness.