For never in the history of the world would an addition of seven and five cease to make twelve, and never have the principles of geometry been falsified in experience. Here is an instance of knowledge independent of sensations. Kant is here a dogmatist, for instead of asking whether synthetic judgements a priori are possible, he takes for granted that there is already such knowledge, and concerns himself with how synthetic judgements a priori are possible. He is only fired with the zeal for describing the anatomy and demonstrating the working of such knowledge, and considers, as against Hume, that to deny a necessary and universal knowledge would be a mere 'scandal'. For Kant all knowledge is in the form of judgements.
Genuine knowledge is a necessary and universal judgement.
Sensations have nothing of the necessary or the universal in them. 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.