Locke confined knowledge to sensory information and the workings of the mind, and he had a moderate skepticism about claims beyond those two sources of information. Locke introduced his Essay Concerning Human Understanding (1689) as the result of conversations among friends which led to the question of what it was possible for them to know, given the limitations of human faculties: “It was necessary to examine our own abilities, and see what objects our understandings were or were not fitted to deal with.” Locke’s method was not to rely on tradition or what other philosophers had claimed, but to look to “the things themselves.”

Knowledge, according to Locke, was direct awareness of some fact. The only facts we can know are those that consist of relationships among our ideas. A fact is something true about the world. Locke did not think that we had direct experience of the world. Things in the world acted on our sense organs to produce ideas. Therefore, the truths we know (facts) are about the relationships between ideas. Ideas are mental objects for Locke, some of which are representations of things in the world. In Book I of the Essay, Locke attacks the rationalist doctrine of innate ideas and innate knowledge. His argument is that we have innate capacities, but nothing like knowledge until there is experience—this is Locke’s famous description of the mind as a tabula rasa, or blank slate.

In Book II, he explains our different types of ideas by tracing them to sensation and reflection on sensation. Reflection consists of combination, division, generalization, and abstraction. For Locke, our ideas are like impressions from experience. When we consider our ideas in our minds, we can combine different ideas, divide an idea into more ideas, generalize about what ideas in a group share, or abstract some property shared by a group of ideas. In Book III, Locke explains how words can mislead us about facts or “the things themselves.” Book IV is a discussion of how we are obligated to conduct our minds in forming beliefs, so as not to stray too far from what we know.