Summary: An Essay Concerning Human Understanding

The understanding, which gives humans their superiority over animals, is like the eye: it enables us to see, yet it does not naturally perceive itself.
Let us attempt to reverse our perspective and make the understanding itself the object of our inquiry. Perhaps this will enable us to set down any Measure of the Certainty of our Knowledge ¹.

Locke does not, in pursuing this aim, examine the nature of the brain from a physicalist or materialist standpoint. Rather, his goal is to identify the various faculties of the mind and the processes by which our ideas are formed.
In doing so, we may discern the limits of knowledge and, consequently, distinguish between a domain where truth can be attained and one where it remains beyond our reach.
For Locke, this is the most effective way to counter scepticism, which calls into question the very possibility of truth. The approach consists in refining radical doubt—determining which ideas are rightly subject to it and which can withstand it.

Locke clarifies what he means by "idea" in a general sense: whatsoever is the Object of the Understanding when a Man thinks ².

Book I: Innate Notions

In the first book, Locke challenges the doctrine of innate ideas, as defended by Descartes. This doctrine holds that humans are born with certain pre-existing ideas—such as the idea of God, as argued in Meditations on First Philosophy.

Locke argues that human beings can arrive at all ideas through the simple use of their natural faculties. We are not born with the idea of red, for instance, but acquire it through sight.

And yet certain principles are universally recognised. Might this not suggest that they owe their universality to being innate?

Locke questions the very existence of universal principles. Even a tautology such as "what is, is" is unknown to a large portion of humanity—children, for example.

A possible objection holds that such principles are innate in the soul, but that the soul does not perceive them and they remain unrecognised.

Locke counters this by arguing that to claim an idea is innate is to say that the soul naturally perceives it—this is the true meaning of the doctrine. It follows that no innate idea could go unnoticed.

Ultimately, the only point Locke concedes to innatism is that the faculty of understanding itself is innate.

Ideas are formed in our minds through the following processes, in chronological order:

- The senses give us access to the world, and ideas thereby arise in our minds.
- As these ideas become more familiar, they are stored in memory, and we assign names to them.
- The mind abstracts new ideas from those provided by the senses—these are general concepts.
- The mind then reasons upon these concepts and derives further ideas.

Locke devotes an entire chapter to practical principles in order to show that none are universal and, therefore, none are innate. If morality were innate, we would all be moral, and we would all feel remorse in cases of murder or theft alike—which is plainly not the case. Moral rules must be justified, and what requires justification cannot be innate.

Locke draws on a classic argument from scepticism, pointing to the diversity of moral customs across societies: child sacrifice among the Greeks and Romans, the abandonment of the elderly in certain tribes, and so on.

In reality, we regard practical principles as innate because we have either never known or have long forgotten their origins. On closer inspection, Doctrines, that have been derived from no better original, than the Superstition of a Nurse, or the Authority of an old Woman; may, by length of time, and consent of Neighbours, grow up to the dignity of Principles in Religion or Morality ³.

¹ An Essay concerning Human Understanding, Clarendon Oxford University Press, Oxford, 1975, I, 1, §2, p.44
² §8, p.47
³ Book I, 3, §22, p.81