Empiricist Theory of Ideas

As an empiricist, Lock believes that all of our knowledge comes from experience. He further holds that all of our knowledge is built from ideas (think of ideas as little building blocks and knowledge as the structures we create out of them). Taking these two commitments together, he concludes that all of knowledge can be accounted for by accounting for the origin of our ideas. Therefore, Book 2, which is all about Locke's theory of ideas, is perhaps the most important part of the Essay Concerning Human Understanding.

According to Locke there are two and only two sources for all the ideas we have. The first is sensation, and the second is reflection. In sensation, much as the name suggests, we simply turn our senses toward the world and passively receive information in the form of sights, sounds, smells, and touch. In this way, we receive such ideas as "blue," "sweet," and "loud." In reflection, on the other hand, we turn our mind on itself, and, again passively, receive such ideas as "thought," "belief," "doubt," and "will."

Perhaps the most important issue regarding Locke's theory of ideas is the question of what role an idea is supposed to play in the act of perception. According to the way most people understand Locke, the idea is actually the object of perception. A tree in the external world causes an idea, and this idea, not the tree itself, is what I perceive. This might seem very strange; it is natural to assume that when I have a perception of a tree the object of my perception is the tree. Nonetheless, the majority of philosophers have taken Locke to be saying just this, and there is much evidence to support them. This view of ideas, called the veil of perception because it posits a veil of ideas between us and the world, is still held by many contemporary philosophers of mind.