Kant asks what we can know, and how we can know it.

This is one of the central questions of philosophy—the theory of knowledge or cognition ("epistemology").

One derive the greater part of our knowledge from observing the real world. From an early age, we see things, we listen, we touch, and so on. Gradually, we build up a picture of the world in which we live. This kind of knowledge is the knowledge of sense-perception. For empiricists like Locke, there is no other kind. Here Kant disagrees. In getting to know the world, the mind is not merely an empty vessel, which can be filled with any content (Locke described it as a tabula rasa—a blank slate).

For Kant, the act of cognition is not passive, but active. We do not simply make a list of the things we see, but consciously select, order and interpret them. For this, the mind has its own method and rules. There are forms of thought which we apply, consciously or unconsciously, when we attempt to understand the information provided by our senses ("sense data").

Kant argues that while most knowledge is derived from experience, part of our knowledge is a priori, and not derived from experience.

In Kant’s opinion, we can only know what is given to us in sense experience. However, the things in themselves, which cause our sensations, cannot be known.

Here, Kant is skating on thin ice. Although he denied it, these views seem to be similar to the subjective idealism of Hume and Berkeley.

Kant changed some of his formulations in the second edition, precisely to avoid this conclusion. In the first edition, he implied that the thinking subject might be the same thing as the object which it perceives. Later, he changed this, maintaining that things outside ourselves certainly exist, but they manifest themselves to us only in appearance, not as they are in themselves.

According to Kant, there are some ideas which are not derived from sense-perception. This shows the difference between the philosophy of Kant and that of Locke, who held that all knowledge whatsoever came from the senses. By contrast, Kant claimed that some knowledge was inborn, namely, the knowledge of space and time. If we make abstraction from all physical aspects of phenomena, he says, we are left with just two things—time and space. Now time and space, together with motion, are the most general and fundamental properties of matter. The only way that it is possible to understand them is in relation to material things. But Kant was an idealist. He insisted that the notions of time and space were inborn. They did not come from experience but were what he called a priori (from the Latin meaning "from the beginning").

To support his idea that space and time are a priori phenomena, Kant uses a very peculiar mode of reasoning. He maintains that, whereas it is impossible to think of objects without time, it is quite possible to think of the time without objects; the same in relation to space. In point of fact, space and time are inseparable from matter, and it is impossible to conceive of them as "things in themselves."

Kant states that it is possible to imagine space with nothing in it, but impossible to imagine no space. But this is not so.

Kant's view of knowledge presents a synthesis of experiences either in present or 'a priory'

Space without matter is just as much an empty abstraction as matter without space. In point of fact, time, space and motion are the mode of existence of matter, and can be conceived of in no other way. Kant’s idea that time and space are outside the range of sense-experience has been refuted by the discoveries of non-Euclidian geometry.

*In Anti Duhring, Engels shows that the whole concept of a priori knowledge is false. All ideas are ultimately derived from reality, even the axioms of mathematics.

It is true that, if we leave aside all the material qualities of a thing, all that is left is space and time. However, these are now empty abstractions.

They cannot stand on their own, any more than there can be fruit, without apples, pears, oranges etc.; or humanity, without human beings, and so on.

The only difference is that the idea of fruit, or humanity, are abstractions of a particular kind of matter, whereas time and space are the most general features, or, more correctly, the mode of existence, of matter in general.*

Reference-

http://educationalphilosophy.blogspot.com/2008/05/kants-theory-of-knowledge.html