If Aristotle for a moment grasped the important truth, that the laws of things and the laws of thought were alike objects of metaphysical inquiry, the conviction produced hardly any result in the details of his treatment: his psychology allied itself chiefly to physics: his metaphysics, after its introductory chapter, deserted the track of psychology.
If Locke laid the foundation of a better method of metaphysical inquiry, when he declared that “ before we set ourselves on inquiries of this nature, it was necessary to examine our own abilities, and see what objects our understandings were or were not fitted to deal with,” he prematurely excluded the very question which his method was required to solve, by asserting that we have no ideas of body or spirit as substances, but merely suppose an unknown substratum to our external or internal ideas.
The barrier thus interposed between the sister streams of thought was widened as each flowed on: the ontological philosophers of modern Germany gloried in being not merely independent of, but even contradictory to, the testimony of consciousness, while the psychological teachers of France and Britain confined themselves more and more within the charmed circle of phenomena, till D’Alembert declared that the office of metaphysics was to prove that all our ideas come from sensation; J and Stewart denounced the inquiries of ontology as “ the most idle and absurd speculation that ever employed the human faculties.”
But the principle on which this conclusion logically rests is, as regards the Scottish philosophy, an excrescence rather than an integral portion of the system. We may refuse to admit the improved dogma which denies to the human mind any conception of substance, and yet avail ourselves of the psychological researches of Reid and Stewart, as a valuable, if an incomplete contribution to the philosophy of consciousness, and through that, to the solution of those fundamental problems of metaphysics to which consciousness gives rise.
***Excerpt from Henry Longueville Mansel, B.d.. Metaphysics or the philosophy of consciousness