Locke's An Essay Concerning Human Understanding is an epistemology, or a study of the phenomenon of human knowledge and belief.
In Book I, he turns to the question of nature vs nurture, and he argues against the idea that a human is born with a set of innate ideas or beliefs. Rather, he argues that humans are born without ideas or thoughts, but that through experience, they construct their worldview one moment at a time.
In Book II, Locke outlines a division between passive and active learning, stating that beliefs that come into the mind from passive methods tend to be more simply, whereas complex ideas, especially abstract ideas like numbers and math, are acquired with active participation, with effort. He also outlines another division, a division between qualities, saying that primary qualities can be known as truly extant, where as secondary qualities are more like what the object represents to the subjective experience of the person. In other words, an apple is a red object, but to a human, it's food.
In Book III, Locke turns to the question of language and the way language shapes the mind. Locke describes the way words signify ideas, and he makes the claim that man is unique in his linguistic abilities. (Trying to specify what the essential difference between mankind and the animal kingdom was very popular during this season of philosophy).
In Book IV, the final chapter of the essay, the subject of the essay broadens to discuss the nature of knowledge, offering a true ontology along with his epistemology. By saying a thing can be known, Locke argues that they have made an ontological assumption, but it's not clear that humans have the objectivity to make those kinds of truth claims, so ultimately, human intuition is assumption. He ends the work outlining the various categories for the sciences.
