Summary: An Essay Concerning Human Understanding (page 7)

Locke aims to define the limits of our knowledge. Our ignorance stems from a lack of ideas, an absence of discernible connections between them, or a failure to examine them closely.

He returns to the objection: if our knowledge concerns only our ideas, how can we ascertain whether things truly exist in the external world as the foundation of these ideas? Perhaps, as in a dream, we are merely confronted with fictitious representations that correspond to nothing real?

Is the knowledge Locke describes not merely a kind of Castle in the Air ¹?

In reality, there exists a mediate knowledge of things, which we access through our ideas.

Two kinds of ideas certainly correspond to reality:

- Simple ideas, since, as we saw in Book II, the mind cannot generate them on its own but receives them passively through external experience.

As a result, simple Ideas are not fictions of our Fancies, but the natural and regular productions of Things without us, really operating upon us ².

- Our complex ideas, with the exception of those of substances, are neither true nor false. They need not conform to external things, since they are archetypes produced by the mind itself.

Thus, that which is not designed to represent any thing but it self, can never be capable of a wrong representation ³.

What distinguishes these complex ideas from chimeras is that they function as archetypes: things themselves seek to conform to these ideas, and so the agreement between ideas and things is guaranteed.

Locke uses the example of a triangle: even if no triangle actually exists, it remains true that its three angles are equal to two right angles.

This would hold of any triangle that did exist: the mathematician is sure what he knows concerning those Figures, when they have barely an Ideal Existence in his Mind, will hold true of them also, when they have a real existence in Matter ⁴.

Similarly, our moral ideas are archetypes, and for this reason moral knowledge can attain the same degree of certainty as mathematics.

With complex ideas, then, if things were to exist, they would necessarily conform to the idea; ideas and things are therefore always in agreement.

This does not apply, however, to ideas of substance. Such ideas refer to an archetype external to the mind, and substances, being referred to Archetypes without us, may differ from them ⁵. The complex idea of a substance may, for instance, include more simple ideas than actually compose it. Ideas of substance may therefore not always correspond perfectly to things themselves—and in practice, they often do not.

Our knowledge of a substance is confirmed when experience reveals that two simple ideas coexist within a thing.

All the simple ideas we observe coexisting in a substance can then be reliably brought together to form abstract ideas of substance, for whatever have once had an union in Nature, may be united again ⁶.

Even if the idea thus formed is true, it may still fall short of a fully accurate representation. Such knowledge is inevitably limited—yet within those limits, it remains genuine knowledge.

Locke goes on to examine certain forms of knowledge, including universal propositions, axioms, and judgement.

He concludes that the knowledge of our own Being, we have by intuition, whereas the existence of a GOD, Reason clearly makes known to us and the Knowledge of the Existence of any other thing we can have only by Sensation ⁷.

This leads him to examine the relationship between belief, faith, and reason.

¹ IV, 4, §1, p.562
² IV, 4, §4, p.564
³ ibid., §5
⁴ IV, 4, §6, p.565
⁵ IV, 4, §11, p.568
⁶ IV, 4, §12, p.569
⁷ IV, 11, §1, p.630