Berkeley argued that the arguments that Locke thought were enough to show some qualities were 'in the mind' were in fact enough to show that all properties were the same in this regard.
Berkeley's gist is to claim that all the arguments Locke uses to show that such and such a quality is secondary apply to all qualities.
Some of the arguments Locke mounts in trying to establish the primary/secondary distinction:
'Let us consider the red and white colours in porphyry. Hinder light from striking on it, and its colours vanish; it no longer produces any such ideas in us: upon the return of light it produces these appearances on us again. Can any one think any real alterations are made in the porphyry by the presence or absence of light; and that those ideas of whiteness and redness are really in porphyry in the light, when it is plain it has no colour in the dark? It has, indeed, such a configuration of particles, both night and day, as are apt, by the rays of light rebounding from some parts of that hard stone, to produce in us the idea of redness, and from others the idea of whiteness; but whiteness or redness are not in it at any time, but such a texture that hath the power to produce such a sensation in us.'
Essay, Bk II, Ch. VIII, Section 19
'The ideas of the primary alone really exist. The particular bulk, number, figure, and motion of the parts of fire or snow are really in them,- whether any one's senses perceive them or no: and therefore they may be called real qualities, because they really exist in those bodies. But light, heat, whiteness, or coldness, are no more really in them than sickness or pain is in manna. Take away the sensation of them; let not the eyes see light or colours, nor the ears hear sounds; let the palate not taste, nor the nose smell, and all colours, tastes, odours, and sounds, as they are such particular ideas, vanish and cease, and are reduced to their causes, i.e. bulk, figure, and motion of parts.'
Locke, Essay, Bk II, Ch. VIII, Section 17.
'Ideas being thus distinguished and understood, we may be able to give an account how the same water, at the same time, may produce the idea of cold by one hand and of heat by the other: whereas it is impossible that the same water, if those ideas were really in it, should at the same time be both hot and cold. For, if we imagine warmth, as it is in our hands, to be nothing but a certain sort and degree of motion in the minute particles of our nerves or animal spirits, we may understand how it is possible that the same water may, at the same time, produce the sensations of heat in one hand and cold in the other; which yet figure never does, that never producing- the idea of a square by one hand which has produced the idea of a globe by another. But if the sensation of heat and cold be nothing but the increase or diminution of the motion of the minute parts of our bodies, caused by the corpuscles of any other body, it is easy to be understood, that if that motion be greater in one hand than in the other; if a body be applied to the two hands, which has in its minute particles a greater motion than in those of one of the hands, and a less than in those of the other, it will increase the motion of the one hand and lessen it in the other; and so cause the different sensations of heat and cold that depend thereon.'
Essay, Bk II, Ch. VIII, Section 21
'Pound an almond, and the clear white colour will be altered into a dirty one, and the sweet taste into an oily one. What real alteration can the beating of the pestle make in any body, but an alteration of the texture of it?'
Essay, Bk II, Ch. VIII, Section 20
(Descartes had argued (among other things): Secondary qualities are only perceptible through a single sense.
Malebranche had argued: the perception of secondary qualities varied with the position of the observer, and so cannot be 'in' the object. E.g. a thing appears to have a different colour depending on the angle of perception, or the light.)
'They who assert that figure, motion, and the rest of the primary or original qualities do exist without the mind in unthinking substances, do at the same time acknowledge that colours, sounds, heat, cold, and suchlike secondary qualities, do not - which they tell us are sensations existing in the mind alone, that depend on and are occasioned by the different size, texture, and motion of the minute particles of matter. This they take for an undoubted truth, which they can demonstrate beyond all exception. Now, if it be certain that those original qualities are inseparably united with the other sensible qualities, and not, even in thought, capable of being abstracted from them, it plainly follows that they exist only in the mind. But I desire any one to reflect and try whether he can, by any abstraction of thought, conceive the extension and motion of a body without all other sensible qualities. For my own part, I see evidently that it is not in my power to frame an idea of a body extended and moving, but I must withal give it some colour or other sensible quality which is acknowledged to exist only in the mind. In short, extension, figure, and motion, abstracted from all other qualities, are inconceivable. Where therefore the other sensible qualities are, there must these be also, to wit, in the mind and nowhere else.'
'I shall farther add, that, after the same manner as modern philosophers prove certain sensible qualities to have no existence in Matter, or without the mind, the same thing may be likewise proved of all other sensible qualities whatsoever. Thus, for instance, it is said that heat and cold are affections only of the mind, and not at all patterns of real beings, existing in the corporeal substances which excite them, for that the same body which appears cold to one hand seems warm to another. Now, why may we not as well argue that figure and extension are not patterns or resemblances of qualities existing in Matter, because to the same eye at different stations, or eyes of a different texture at the same station, they appear various, and cannot therefore be the images of anything settled and determinate without the mind?
'Again, it is proved that sweetness is not really in the sapid thing, because the thing remaining unaltered the sweetness is changed into bitter, as in case of a fever or otherwise vitiated palate. Is it not as reasonable to say that motion is not without the mind, since if the succession of ideas in the mind become swifter, the motion, it is acknowledged, shall appear slower without any alteration in any external object?'
Berkeley's summing up:
'In short, let any one consider those arguments which are thought manifestly to prove that colours and taste exist only in the mind, and he shall find they may with equal force be brought to prove the same thing of extension, figure, and motion. Though it must be confessed this method of arguing does not so much prove that there is no extension or colour in an outward object, as that we do not know by sense which is the true extension or colour of the object. But the arguments foregoing plainly shew it to be impossible that any colour or extension at all, or other 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.'
(Principles of Human Knowledge, Section 15)
If a sensible object is an idea (or set of ideas) what becomes of it when it is not being perceived?
Two shots: (a) when we say such-and-such exists even though it is not being perceived we mean that such-and-such perceptions would be got under certain conditions. (b)a sensible object often exists in the mind of God when it is not before the mind of one of us.
There was once a young man who said 'God
Must find it exceedingly odd
If he finds that this tree
Continues to be
When there's no one about in the Quad.'
Dear Sir, Your astonishment's odd;
I am always about in the Quad.
And that's why the tree
Will continue to be,
Since observed by yours faithfully, God.
In terms of the example of the general idea of a triangle:
Locke:you get a general idea of triangle by abstracting from a plurality of particular ideas of this triangle, that triangle, etc.
Berkeley: How can there be an idea of a triangle which doesn't have any particular size? Locke's idea of a general idea is a contradiction in terms. In fact we do not have any general ideas. We have particular ideas which are used in a distinctive way.
Natural philosophy is, in Berkeley's view, 'just the study of the uniformities and regularities of our experience.' (Woolhouse, Introduction to Berkeley's Principles of Human Knowledge, p.21.)
Berkeley sometimes expresses his immaterialism in terms of a rejection of something he calls 'substance'.
His gist is: it is nonsensical to posit a something that is completely inaccessible to sense.
How can we have reason to think there is more than one mind (my own)?