Assuming that empirical experience is contingent and non-pure in nature, Kant concludes that pure a priori principles are indispensable in the process of knowing. If, for example, causality is a concept that we use, not because our experience has a certain character, but because it makes objects of a certain sort, and their relations possible for us, then it has necessity for us; it is what we use to constitute an objective world, and so necessarily relative to our standpoint. It is this necessity and universality, and the objective sufficiency ensuing from them that constitute the certainty associated with a priori in the Critique. All synthetic a priori propositions for Kant rest on the structure of the human mind, which, as he believes, has the basic function of synthesising what is given in sense experience; this is a process of ordering the given according to the forms of perception (space and time) and the categories of thinking, both of them being the contributions of the mind. Given this structure of the mind, it can formulate concepts and statements, which are synthetic (ampliative) and a priori (in advance to sense experience) in relation to the forms of thought. Kant’s thrust on the synthetic a priori is motivated by his ultimate aim of transcendental philosophy, namely, establishing the a priori and unchanging elements of morality.
(Source: BPY008/Block 4/Unit 1/Page 15)