Summary: A Treatise Concerning the Principles of Human Knowledge (page 2)

Suppose that a geometer draws a line on a geometrical figure.

It may be said that this line, which in it self is a particular Line, is nevertheless with regard to its signification General, since as it is there used, it represents all particular Lines whatsoever; so that what is demonstrated of it, is demonstrated of all Lines, or, in other Words, of a Line in General ¹.

In short, an idea becomes general by being made a Sign ².

Abstract ideas are not necessary for language—nor for communication, since animals do not use them either, yet they communicate with one another.

Berkeley's critique of abstract ideas frees us from the Labyrinths of Error ³ of scholasticism and the endless, fruitless controversies it has generated.

The source of this error lies in language; we must therefore strive to think of the pure idea, independently of words, so as to avoid purely verbal controversies.

The objects of human knowledge are:

- Ideas imprinted on the senses (sensation)
- Ideas perceived through reflection on our inner states
- Ideas formed with the help of imagination or memory

The ideas of sensation include light, colours, and so on.

Because several of them are often observed together, they are taken to be one and the same thing and grouped under a single name. For example, certain collections of Ideas constitute a Stone, a Tree ⁴, and so on.

Now, we notice that besides all that endless variety of Ideas or Objects of Knowledge, there is likewise something which knows or perceives them, and exercises divers Operations, as Willing, Imagining, Remembering about them. This perceiving, active Being is what I call Mind, Spirit, Soul or my Self ⁵.

This mind is not itself an idea but a thing intirely distinct from them, wherein they exist, or, which is the same thing, whereby they are perceived; for the Existence of an Idea consists in being perceived. ⁶ This last equivalence is fundamental to Berkeley's thought, resting as it does on his celebrated doctrine that to be is to be perceived (esse est percipi ⁷). He further specifies: The Existence of an Idea consists in being perceived ⁸.

What, indeed, do we mean by "existence" when we say that a thing exists?

The Table I write on, I say, exists, that is, I see and feel it; and if I were out of my Study I should say it existed, meaning thereby that if I was in my Study I might perceive it [...] There was an Odor, that is, it was smelled; There was a Sound, that is to say, it was heard; a Colour or Figure, and it was perceived by Sight or Touch ⁹.

This is the full extent of what can be understood by the word "exist".

If to be is to be perceived, the consequences are far-reaching: thoughts, passions, ideas of the imagination, and sensations cannot exist otherwise than in a Mind perceiving them ¹⁰.

This leads us to a radical idealism, in which we abandon the common-sense notion that there is an external world where things exist independently of man:

For as to what is said of the absolute Existence of unthinking Things without any relation to their being perceived, that seems perfectly unintelligible. Their Esse is Percipi, nor is it possible they should have any Existence, out of the Minds or thinking Things which perceive them ¹¹.

In everyday thinking, we usually distinguish between the idea of a thing and the thing itself, existing outside and independently of our minds. Berkeley challenges this Opinion strangely prevailing amongst Men, that Houses, Mountains, Rivers, and in a word all sensible Objects have an Existence Natural or Real, distinct from their being perceived by the Understanding ¹².

This idea involves a manifest Contradiction. For what are the forementioned Objects but the things we perceive by Sense, [...] Is it not plainly repugnant that [objects] should exist unperceived? ¹³.

¹ ibid.
² ibid.
³ §17
⁴ I, §1
⁵ §2
⁶ ibid.
⁷ §3
⁸ §2
⁹ §3
¹⁰ ibid.
¹¹ §3
¹² §4
¹³ ibid.