Hence genuine knowledge must be inherent in the very constitution of the understanding or mind itself, the very make-up of the mind, the necessary and fundamental law which determines the manner of all the functions of the mind. The mind is not a blank tablet, as Locke thought, not a passive recipient of sensations, but an active agent which modifies the form of the sense-material, gives it a different shape, casts it in the mould of order, unity and method, and reorganises its constitution. So in our knowledge we have material from the senses, unity and order from the mind or the understanding. The whole of his 'Critique of Pure Reason' is an attempt to answer this great question. Kant observes that sensations by themselves are subjective states and have to be referred to space and time in order to acquire the character of objectivity in knowledge.
Sensations provide matter, and space and time the form.
In our processes of knowledge we first organise sensations by the application of the perceptual categories of space and time, and then again organise these perceptions by the application of the conceptual categories, the pure concepts and judgements, which are twelve in number. Without the aid of space and time there can be no perception, for sensations independently give us no knowledge of any object. Space and time are the a priori modes or ways of perception, and can also by themselves become contents of pure perception independent of objects. They are a priori , because they are the conditions necessary for the formation of sensations into perceptions. And as the laws of mathematics are the laws of space and time, they are a priori laws.