For Locke, all knowledge comes exclusively through experience. He argues that at birth the mind is a tabula rasa, or blank slate, that humans fill with ideas as they experience the world through the five senses. Locke defines knowledge as the connection and agreement, or disagreement and repugnancy, of the ideas humans form. From this definition it follows that our knowledge does not extend beyond the scope of human ideas. In fact, it would mean that our knowledge is even narrower than this description implies, because the connection between most simple human ideas is unknown.
Because ideas are limited by experience, and we cannot possibly experience everything that exists in the world, our knowledge is further compromised. However, Locke asserts that though our knowledge is necessarily limited in these ways, we can still be certain of some things. For example, we have an intuitive and immediate knowledge of our own existence, even if we are ignorant of the metaphysical essence of our souls. We also have a demonstrative knowledge of God’s existence, though our understanding cannot fully comprehend who or what he is.
We know other things through sensation. We know that our ideas correspond to external realities because the mind cannot invent such things without experience. A blind man, for example, would not be able to form a concept of colour. Therefore, those of us who have sight can reason that since we do perceive colours, they must exist.