Summarise the argument in the following passage. Analyse and critically assess its implications.
‘But, say you, surely there is nothing easier than for me to imagine trees, for instance, in a park, or books existing in a closet, and nobody by to perceive them. I answer, you may so, there is no difficulty in it; but what is all this, I beseech you, more than framing in your mind certain ideas which you call books and trees, and the same time omitting to frame the idea of anyone that may perceive them? But do not you yourself perceive or think of them all the while? This therefore is nothing to the purpose; it only shows you have the power of imagining or forming ideas in your mind: but it does not show that you can conceive it possible the objects of your thought may exist without the mind. To make out this, it is necessary that you conceive them existing unconceived or unthought of, which is a manifest repugnancy. When we do our utmost to conceive the existence of external bodies, we are all the while only contemplating our own ideas. But the mind, taking no notice of itself, is deluded to think it can and does conceive bodies existing unthought of or without the mind; though at the same time they are apprehended by, or exist in, itself. A little attention will discover to anyone the truth and evidence of what is here said, and make it unnecessary to insist on any other proofs against the existence of material substance.’
Taken from:
Berkeley, G. A Treatise concerning the Principles of Human Knowledge (1710) in T.E. Jessop (ed.) Berkeley, Philosophical Writings (Austin TX: University of Texas Press, 1953), paragraph 23.