"All the choir of heaven and the furniture of the earth, in a word, all those bodies which compose the mighty frame of the world, have not any subsistence outside the mind, and their being ... is to be perceived or known; that consequently so long as they are not actually perceived by me, or do not exist in the mind or that of any other created spirit, they must either have no existence at all, or else subsist in the mind of some eternal spirit.
From what has been said, it follows, there is not any other substance than spirit, or that which perceives. (6-7)"
"I am content to put the whole upon this issue: If you can but conceive it possible for one extended movable substance, or, in general, one idea, or anything like an idea, to exist otherwise than in a mind perceiving it, I shall readily give up the cause.
But, say you, 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 at the same time omitting to frame the idea of any one that may perceive them? But do you not yourself perceive or think them all the while? (22-23)"
After the constant conjunction of two objects, heat and flame for instance, weight and solidity, we are determined by custom alone to expect the one from the appearance of the other.
All inferences from experience, therefore, are effects of custom, not reasoning.
Having found, in many instances, that any two kinds of objects, flame and heat, snow and cold, have always been conjoined together; if flame or snow is presented anew to the senses, the mind is carried by custom to expect heat or cold ... All these operations are a species of natural instincts. (736)