In my last Lecture, Gentlemen, I illustrated, at great length, the relation which the Philosophy of Mind bears to all the other sciences, as the common centre of each. These sciences I represented, as, in their relation to the powers of discovery, that are exercised in them, truly arts, in all the various intellectual processes of which, the artist is the same, and the instruments the same; and as to the perfection of any of the mechanical arts, it is essential, that we know the powers of the instruments employed in it, so, in the inventive processes of science of every kind, it seems essential to the perfection of the process, that we should know, as exactly as possible, the powers and the limits of these intellectual instruments, which are exercised alike in all,--that we may not waste our industry, in attempting to accomplish with them what is impossible to be accomplished, and at the same time may not despair of achieving with them any of the wonders to which they are truly adequate, if skilfully and perseveringly exerted; though we should have to overcome many of those difficulties which present themselves, as obstacles to every great effort, but which are insurmountable, only to those who despair of surmounting them. It was to a consideration of this kind, as to the primary importance of knowing the questions to which our faculties are competent, that we are indebted for one of the most valuable works in our science, a work, which none can read even now, without being impressed with reverence for the great talents of its author; but of which it is impossible to feel the whole value, without an acquaintance with the verbal trifling, and barren controversies, that still perplexed and obscured intellectual science at the period when it was written. The work to which I allude is the _Essay on the Human Understanding_, to the composition of which Mr Locke, in his preface, states himself to have been led by an accidental conversation with some friends who had met at his chamber. In the course of a discussion, which had no immediate relation to the subject of the Essay, they found themselves unexpectedly embarrassed by difficulties that appeared to rise on every side, when after many vain attempts to extricate themselves from the doubts which perplexed them, it occurred to Mr Locke, that they had taken a wrong course,--that the inquiry in which they were engaged was probably one which was beyond the reach of human faculties, and, that their _first_ inquiry should have been, into the nature of the understanding itself, to ascertain what subjects it was fit to explore and comprehend.