George Berkeley, answered from the texts and cited to the page.
I challenge the matterist to conceive a sensible object existing unperceived.1 Take the question concretely. I am hearing this A sharp; can I conceive it existing unheard? I cannot, for if it were unheard it would not be A sharp. I am seeing this pink patch; can I conceive it existing unseen? I cannot, for if it were unseen it would not be a pink patch.
The matterist confines himself, in this question, to actual perception, and finds that within actual perception the supposed unperceived existence cannot be framed. He may retreat to imagined perception: surely I can imagine a tree in a forest where no one is. Try the case carefully. You have framed a mental picture of a tree; you have placed the tree in a forest; you have specified that no one is there.
But there is an observer in the picture — namely you, the one imagining. Imagined existence is imagined perception, and the observer you tacitly placed in the scene is yourself. You have not conceived an unperceived object; you have conceived an object you are imagining, and the observer is built into the imagining. The argument is not a logical fallacy or a verbal trick; it is an empirical challenge to the supposed conception.
Try it for yourself. Whoever says he can conceive a sensible object existing unperceived must either have framed something other than a sensible object, or must have placed an observer in the scene without counting himself among the observers.2 Absolute existence of unthinking sensible things is words without meaning, or which include a contradiction.345
As I was thinking of a tree in a solitary place, where no one was present to see it, methought that was to conceive a tree as existing unperceived or unthought of, not considering that I myself conceived it all the while. But now I plainly see, that all I can do is to frame ideas in my own mind. I may indeed conceive in my own thoughts the idea of a tree, or a house, or a mountain, but that is all.Works Vol 2 - Principles and Three Dialogues, pp. 210–212
Pray what think you of this? It is just come into my 30 head, that the ground of all our mistakes lies in your treating of each quality by itself. Now, I grant that each quality cannot singly subsist without the mind. Colour cannot without extension, neither can figure without some other sensible quality.Works Vol 2 - Principles and Three Dialogues, pp. 209–210
That is, suffer me to serve you in your own kind, and I warrant it shall conduct you through as many perplexities and contradictions, to the very 30 same state of scepticism that I my self am in at present. puiLonous, I assure you, Hylas, I do not pretend to frame any hypothesis at all. I am of a vulgar cast, simple enough to believe my senses, and leave things as I find them.Works Vol 2 - Principles and Three Dialogues, pp. 239–240
And lastly whether, in case I granted all you contend for, it would make any thing to your purpose, it not being easy to conceive how jo the external or absolute existence of an unthinking substance, distinct from its being perceived, can be inferred from my allowing that there are certain things perceived by the mind of God, which are to Him the occasion of producing ideas in us.Works Vol 2 - Principles and Three Dialogues, pp. 230–231
For the present, I have no inclination to the answering part. I would much rather see how you can get over what follows. Pray are not the objects perceived by the senses of one, likewise perceivable to others present? Ifthere were an hundred more here, they would all see the garden, the trees, and flowers as I see them. But they are not in the same manner affected with the ideas I frame in my imagination.Works Vol 2 - Principles and Three Dialogues, p. 257
The most-discussed and most-attacked Berkeley argument in twentieth-century philosophy. Russell in the History of Western Philosophy framed the canonical objection: that Berkeley conflated "is conceived" with "is in the mind" and thereby committed a logical fallacy. G.E. Moore's "Refutation of Idealism" (1903) made a related charge. A.J. Ayer analyzed the structure in The Foundations of Empirical Knowledge (1940). Pitcher's Berkeley (1977) reconstructed the argument with care and identified what is and is not defensible in it. Luce's Berkeley's Immaterialism (1945) ch. VIII offered the standard Berkeley-friendly defense — that the argument is empirical, not transcendental. Bennett's Locke Berkeley Hume: Central Themes (1971) provided the most rigorous analytical engagement. The argument has not been settled. Whether it succeeds depends on whether the challenge to "conceive an unperceived sensible object" is taken as a logical demand or an empirical one.