John Locke’s most famous work, An Essay Concerning Human Understanding, deals with fundamental questions regarding the mind, thought, language, and perception, and is broken up into four books. In Essay, Locke provides a systematic philosophy that attempts to answer the question of how we think. As a result of his work, Locke shifted the philosophical dialogue away from metaphysics and toward epistemology.
Locke opposes the notion set forth by other philosophical schools (such as those of Plato and Descartes) that one is born with innate, fundamental principles and knowledge. He argues that this idea would mean all humans universally accept certain principles, and since there are no universally accepted principles (and if there were, they would not be the result of innate knowledge), this cannot be true.
For example, people differ in moral ideas, so moral knowledge cannot be innate. Instead, Locke believed that humans are a tabula rasa, or blank slate, that gain knowledge through experience. The experience creates simple ideas (based on the senses, reflection, and sensation), and as these simple ideas combine, they become more complex (through comparison, abstraction, and combination) and form knowledge. Ideas can also be divided into two categories:
- Primary (which cannot be separate from the matter and are present regardless of whether a person sees them or not—for example, size, shape, and motion)
- Secondary (which are separate from the matter and are only perceived when the matter is observed—for example, taste and odor)
Lastly, Locke objects to Plato’s concept of essences, the notion that humans can only identify an individual to be part of a species because of its essence. Locke creates his own theory of essences based on observable properties (which he calls nominal essences) and the invisible structures that form the observable properties (which he calls real essences). For example, we can form an idea and create an essence about what a dog is based on what we observe and based on the biology of the dog (which is responsible for the observable properties). To Locke, human knowledge is limited, and humans should be aware of such limitations.