Summary: An Essay Concerning Human Understanding (page 5)

Locke also distinguishes between real ideas, which have a Foundation in Nature, have a Conformity with the real Being, and Existence of Things, or with their Archetypes ¹, and chimerical ideas.

This applies to our simple ideas, which are all real [and] all agree to the reality of things ², since the mind cannot produce simple ideas but can only receive them passively.

It also applies to secondary qualities, which, though not inherent in things, result from a power that truly resides in the object itself.

With complex ideas, matters become more complicated. The mind may combine simple ideas incorrectly, in which case the resulting complex idea is chimerical. Conversely, complex ideas formed from compatible simple ideas are real.

Similarly, ideas of substances are real only when they consist of simple ideas that can genuinely be combined. The Centaur, for instance, is chimerical: the head of a bull cannot be joined to a man's body.

Locke returns to the question of the reality of our ideas in Book IV.

Ideas are complete or incomplete depending on whether they represent all or only part of the real original, or archetype, to which they correspond. Our simple ideas and modes are complete; substances, however, lack this completeness.

Book III: Of Words

Human beings use Sounds as Signs of internal Conceptions, [...] to make them stand as marks for the Ideas within his own Mind, whereby they might be made known to others ³. This is the origin of language.

It would be impossible to give a distinct name to everything, given the limits of memory. This is why names are general: they refer to general ideas, and All (except proper) Names are general, and so stand not particularly for this or that single Thing, but for sorts and ranks of Things ⁴.

Words are the sensible marks of ideas, and reflect the thoughts of those who use them.

How are the general ideas corresponding to words formed? Through abstraction: Ideas become general, by separating from them the circumstances of Time, and Place, and any other Ideas, that may determine them to this or that particular Existence ⁵.

A general idea designates what is common to all the particulars it covers.

By tracing the origin of general ideas and the mechanism of their formation through abstraction, Locke aims to challenge Platonism, which asserts the existence of ideas in a separate reality.

In a move akin to nominalism, he asserts that General and Universal belong not to the real existence of Things; but are the Inventions and Creatures of the Understanding, made by it for its own use, and concern only Signs ⁶.

We should therefore not take seriously this whole mystery of Genera and Species, which make such a noise in the Schools, and [...] is nothing else but abstract Ideas, more or less comprehensive, with names annexed to them ⁷.

The genus and species of a thing constitute its nominal essence, which Locke contrasts with its real essence.

This well-known distinction can be summarised as follows:

- Real essence is the internal, true, and unknown constitution of a thing, upon which its discoverable qualities depend.

- Nominal essence, by contrast, refers to the genus and species of a thing. Socrates is a man: this defines what he is—his essence—and determines his species, designated by a name.

What belongs uniquely to Socrates and to no other man is his real essence.

In the case of simple ideas and modes—such as that of a triangle—real essence and nominal essence coincide; this is not the case, however, for substances.

Real essence is perishable: it vanishes when the thing is destroyed. Nominal essence, by contrast, is imperishable: even if every sheep perishes, the species "sheep" remains unchanged.

¹ II, 30, §1, p.372
² ibid.,§2
³ III, 1, §2, p.402
⁴ III, 1, §6, p.404
⁵ III, 3, §6, p.411
⁶ III, 3 §11, p.414
⁷ III, 3, §9, p.412