We are said to THINK on certain subjects, and this process is confessed to require an intense exertion of our intellectual faculties: but for this operation, the materials have not been clearly specified, nor the manner of the elaboration defined. It has been held, that our thoughts are produced by some mysterious assemblage and arrangement of IDEAS, which the mind or soul performs by a dexterous and imperceptible contrivance; although we are conscious of all our acts of intelligence, and on a moment's consideration it will be evident, that such intelligence would be useless without our consciousness. Mr. LOCKE, whose name can never be mentioned without a grateful recollection for the instruction he has afforded us, and for the candour with which he has recorded the difficulties that obstructed the progress of his inquiries, has employed this ideal system most extensively: but it is evident, that he felt the obscurity of his own definition. In his Introduction to the Essay, p. 5, 6th edition, he says, "Before I proceed on what I have thought on this subject, I must here in the entrance beg pardon of my reader, for the frequent use of the word Idea, which he will find in the following Treatise. It being that term which, I think, serves best to stand for whatsoever is the object of the understanding, when a man thinks. I have used it to express whatever is meant by Phantasm, Notion, Species, OR WHATEVER IT IS, which the mind can be employed about in thinking; and I could not avoid frequently using it." Dr. REID follows nearly in the same track:--"It is a fundamental principle of the Ideal system, that every object of thought, must be an _impression_ or an _Idea_, that is, a _faint copy_ of some preceding impression."--_Inquiry into the Human Mind on the Principles of Common Sense_, 1765, p. 41.