Summary: A Treatise Concerning the Principles of Human Knowledge

Berkeley, an Irish bishop, wrote this work for sceptics and unbelievers.

It is often the way we use our thinking—by raising various problems on a given subject—that leads to scepticism. This is due not to reason itself, but to the misuse we make of it. Hence this famous metaphor:

The far greater Part, if not all, of those Difficulties which have [...] block’d up the way to Knowledge, are intirely owing to our selves. [...] We have first rais’d a Dust, and then complain, we cannot see ¹.

So what are these false principles that we have followed and that have led us astray?

They concern the notion of abstraction and, by extension, abstract ideas. These are considered to be the objects of logic or metaphysics, whereas they are merely fictions. Or, to put it another way: we have no abstract ideas.

Berkeley sums up the doctrine of abstraction as follows:

Qualities [...] do never really exist each of them apart by it self, and separated from all others, but are mix’d [...] and blended together, several in the same Object. But we are told, the Mind being able to consider each Quality singly, or abstracted from those other Qualities with which it is united, does by that means frame to it self abstract Ideas. ².

Berkeley gives an example: if one sees an Object extended, coloured, and moved, the mind distinguishes this compound object into simple ideas and does frame the abstract Ideas of Extension, Colour, and Motion ³. He specifies:

Not that it is possible for Colour or Motion to exist without Extension: but only that the Mind can frame to it self by Abstraction the Idea of Colour exclusive of Extension ⁴.

The abstract idea is supposed to be a general idea that groups together what is common to the things of which it is the idea.

Another example illustrates the astonishing nature of this doctrine: it makes an Idea of Colour in abstract which is neither Red, nor Blue, nor White, nor any other determinate Colour ⁵.

Abstract ideas would even be forged from the most complex beings, such as man. Such a being would have a size and a colour, yet no particular size or colour.

Berkeley presents his famous critique of this doctrine of abstraction.

First, he declares that he does not possess this wonderful Faculty of Abstracting [...] Ideas ⁶.

On the other hand, he acknowledges having an entirely different faculty: that of picturing ideas of particular things and combining them. Thus, he can imagine a man with two heads, but that man will have a particular colour and shape: The Idea of Man that I frame to my self, must be either of a White, or a Black, or a Tawny, a Straight, or a Crooked, a Tall, or a Low, or a Middle-sized Man. ⁷.

In fact, the general abstract idea is an impossibility in itself: I cannot by any effort of Thought conceive the abstract Idea above described. ⁸.

I can separate qualities in thought only if they can be separated in reality (e.g. by imagining a man with one arm missing): I cannot conceive separately, those Qualities which it is impossible should Exist so separated ⁹.

Berkeley opposes Locke, who, in An Essay Concerning Human Understanding, asserts that the faculty of forming abstract general ideas is what makes man superior to animals. He assumes that language implies the possession of general ideas. Yet, according to Berkeley, this is a mistake:

A Word becomes general by being made the Sign, not of an abstract general Idea but, of several particular Ideas, any one of which it indifferently suggests to the Mind. ¹⁰.

In fact, Berkeley does not deny absolutely that there are general Ideas, but only that there are any abstract general Ideas ¹¹.

Here is how he characterises this type of idea: An Idea, which considered in it self is particular, becomes general, by being made to represent or stand for all other particular Ideas of the same sort ¹².

¹ A Treatise Concerning the Principles of Human Knowledge, Introduction, §3
² ibid., §7
³ ibid.
⁴ ibid.
⁵ §8
⁶ §10
⁷ ibid.
⁸ ibid.
⁹ ibid.
¹⁰ §11
¹¹ §12
¹² ibid.