Summary: A Treatise Concerning the Principles of Human Knowledge (page 3)

Where, then, does this error come from? It arises precisely from the doctrine of abstract ideas.

To distinguish the existence of perceived objects from the mere fact of their being perceived is itself a particularly subtle abstraction.

What are colours, if not sensations imprinted upon the senses?

Is it possible to separate, even in thought, any of these from Perception? For my part I might as easily divide a Thing from it Self ¹.

In truth, then, object and sensation are one and the same, and cannot be abstracted from each other.

Berkeley therefore presents it as self-evident that, since all things exist only within a mind and their being consists in being perceived or known, there is not any other Substance than Spirit, or that which perceives. ².

Berkeley adopts the Aristotelian notion of substance, which denotes that which underlies and supports apparent phenomena. Traditionally, however, substance was attributed to matter; here, it is spirit that takes on this role.

But can we not conceive of external things that resemble our ideas, the latter being merely their copies?

Berkeley responds to this objection by arguing that an Idea can be like nothing but an Idea ³. Either these originals are themselves ideas—in which case they are perceived in themselves—or there are no originals at all. Either way, there is no external world composed of mind-independent things.

Berkeley now turns to Locke, and specifically to his celebrated distinction between primary qualities (extension, shape, motion, number, and so on) and secondary qualities (colours, sounds, flavours, and so on).

According to Locke, only primary qualities exist in external bodies, independently of our minds. Secondary qualities exist only in our minds; they are the human interpretation of primary qualities. Thus, colour (a secondary quality) is merely a vibration in space—that is, a movement of a certain extension (a primary quality).

These primary qualities exist independently of human perception and share the common feature of being qualities of matter, defined as an inert, senseless substance in which extension, figure, and motion actually subsist.

Berkeley rejects this conception, and in particular the notion upon which it rests—matter as Corporeal Substance ⁵—which he regards as contradictory.

Once again, he treats this as an abstraction. On Locke's account, matter would in reality be devoid of any secondary qualities—without colour, odour, solidity, and so on—consisting of nothing but extension, shape, and motion. Yet this is inconceivable, like any abstraction.

Moreover, the arguments used to deny the reality of secondary qualities can be applied equally to primary qualities. Heat (a secondary quality) is held to exist only in the mind because what feels cold to one person feels hot to another. By the same token, extension (a primary quality) exists only in the mind, since the same eye, depending on its position, perceives different extensions—a circle seen from above may appear as a line when viewed in profile.

Berkeley therefore concludes:

[It is] impossible that any [...] sensible Quality whatsoever, should exist in an unthinking Subject without the Mind, or in truth, that there should be any such thing as an outward Object. ⁶.

The proponents of matter as corporeal substance, it turns out, have no clear idea of what this notion entails. They define matter as that which supports sensible qualities—as a substance. But what does this "support" mean? Is it to be understood in its usual or literal Sense, as when we say that Pillars support a Building? ⁷ Surely not. They thus have no account of the relation between matter-substance and its accidents.

But how can it be that no external material thing is the cause of our ideas and perceptions? Dreams make this perfectly clear: when we dream, we have perceptions with no external thing as their source: The Supposition of external Bodies is not necessary for the producing our Ideas ⁸.

¹ §5
² §7
³ §8
⁴ §9
⁵ ibid.
⁶ §15
⁷ §16
⁸ §18