'But what if these profound disquisitions into the first principles of human nature, do naturally and necessarily plunge a man into this abyss of scepticism? May we not reasonably judge from what hath happened? Des Cartes no sooner began to dig in this mine, than scepticism was ready to break in upon him. He did what he could to shut it out. Malebranche and Locke, who dug deeper, found the difficulty of keeping out this enemy still to increase; but they laboured honestly in the design. Then Berkeley, who carried on the work, despairing of securing all, bethought himself of an expedient: By giving up the material world, which he thought might be spared without loss, and even with advantage, he hoped by an impregnable partition to secure the world of spirits. But, alas! the Treatise of Human Nature wantonly sapped the foundation of this partition and drowned all in one universal deluge.' (Chapter I, Sections vi-vii.) What Reid so pertinently describes here as the 'enchanted castle' is nothing else than the human head, which knows of no occurrence beyond its boundaries, because it has forgotten that it is only the end-product of a living existence outside of, and beyond, itself. We see here that Reid is gifted with the faculty of entering this castle without forfeiting his memory of the world outside; and so even from within its walls, he could recognize its true nature. To a high degree this helped him to keep free of those deceptions to which the majority of his contemporaries fell victim, and to which so many persons are still subject to-day. It is in this way that Reid could make it one of the cardinal principles of his observations to test all that the head thinks by relating it to the rest of human nature and to allow nothing to stand, which does not survive this test. In this respect the argument he sets over against the Cartesian, 'cogito ergo sum' is characteristic: ' "I am thinking," says he, "therefore I am": and is it not as good reasoning to say, I am sleeping, therefore I am? If a body moves, it must exist, no doubt; but if it is at rest, it must exist likewise.' The following summarizes the position to which Reid is led when he includes the whole human being in his philosophical inquiries. Reid admits that, when the consciousness that has become aware of itself surveys that which lies within its own horizon, it finds nothing else there but transient pictures. These pictures in themselves bring to the mind no experience of a lasting existence outside itself. There is no firm evidence of the existence of either an outer material world to which these pictures can be related, or of an inner spiritual entity which is responsible for them. To be able to speak of an existence in either realm is impossible for a philosophy which confines its attention solely to the mere picture-content of the waking consciousness.