In The Critique of Pure Reason Kant attempted to prove, that although our knowledge is derived from experience, it is possible to have knowledge of objects in advance of experience. Its truth could be known only a posteriori , after it had received validation from experience. An a priori knowledge is knowledge which is independent of all experience. A priori proposition is one that can be known to be true, or false, without reference to experience, except so far as experience is necessary for understanding its terms. An a posteriori knowledge is derived from experience.
All bodies are heavy' is simply a generalization to which no exception have been observed.
But Kant stated that there is a third category, namely synthetic a priori . Kant firmly believed that threre is an independet reality outside the world of all possible experience, calling this the world of the noumenal, the world of things as they are in themselves, and of reality as it is in itself. The world of phenomena was the world of things as they appears to us - the directrly known world of actual experience. He suggested, that the way in which we perceive, identify, and reflect upon objects might have itself a form or structure which in some ways moulds or contributes to our experience. Among them are space, time, quantity, quality, relation, modality, and their subforms.