Summary: An Essay Concerning Human Understanding (page 4)

Locke criticises the scholastic notion of substance:

We have no Idea of what it is, but only a confused obscure one of what it does. ¹

He goes on to argue that whoever examines this idea will find that he has no other Idea of it at all, but only a Supposition of he knows not what support of such Qualities, which are capable of producing simple Ideas in us; which Qualities are commonly called Accidents. [...] The Idea then we have, to which we give the general name Substance, being nothing, but the supposed, but unknown support of those Qualities ².

If we lack a clear idea of the substance of a body, the same holds true of the substance of the mind—namely, the soul.

We cannot conclude, however, merely because we lack a clear idea of them, that they do not exist.

This argument counts against materialism: for while we have no idea of what a spiritual substance is, we equally lack one of a corporeal substance.

Locke reduces good and evil to pleasure and pain: good is whatever promotes pleasure, while evil is whatever causes pain.

In a definition that has remained famous, he describes desire as "inquietude" (uneasiness): desire is the uneasiness a Man finds in himself upon the absence of any thing, whose present enjoyment carries the Idea of Delight with it ³.

It is worth noting, moreover, that the chief if not only spur to humane Industry and Action is uneasiness ⁴.

Indeed, if one could be deprived of what one desires while remaining perfectly content, one would simply not desire it.

Locke goes on to argue that human action is not driven by the highest good, as in the traditional Aristotelian view, but by this uneasiness.

The drunkard knows he is straying from the highest good when he reaches for a drink, yet he is driven by the uneasiness of lacking what he craves most: alcohol.

Locke examines the interplay between the faculties of understanding and will, raising questions such as: "Are we free to will or not?" This leads him to define freedom as a power to act or not to act according as the Mind directs ⁵.

He also explains how particular ideas take shape in our minds—notably, the idea of God.

Through introspection, we derive the ideas of existence, duration, knowledge, power, pleasure, happiness, and so on. We then enlarge every one of these with our Idea of Infinity; and so putting them together, make our complex Idea of God ⁶.

Locke develops his celebrated theory of personal identity.

What constitutes personal identity? What guarantees that I remain the same person despite the changes wrought by time—to my body, my ideas, and so on?

The answer lies in consciousness, which is the foundation of personal identity:

As far as this consciousness can be extended backwards to any past Action or Thought, so far reaches the Identity of that Person; it is the same self now it was then; and 'tis by the same self with this present one that now reflects on it, than that Action was done ⁷.

It is not identity of substance but identity of consciousness that constitutes the self. This is why it would be unjust to punish Socrates while awake for what he did or thought while asleep.

Some ideas are clear—when the mind grasps them immediately and has a full and evident perception ⁸ of them—while others are obscure, when this is not the case.

Some ideas are distinct—when the mind is able to distinguish them from other closely related ideas—while others are confused, when this is not the case.

¹ II, 13, §19, p.175
² II, 23, §2, p.295-296
³ II, 20, §6, p.230
⁴ Ibid.
⁵ II, 21, §71, p.282
⁶ II, 23, §33, p.314
⁷ II, 27, §9, p.335
⁸ II, 29, §4, p.364