The following passage from the first chapter of the Inquiry reveals Reid as a personality who was not dazzled to the same extent as were his contemporaries by the brilliance of the onlooker-consciousness:
'If it [the mind] is indeed what the Treatise of Human Nature makes it, I find I have been only in an enchanted castle, imposed upon by spectres and apparitions. I blush inwardly to think how 1 have been deluded; I am ashamed of my frame, and can hardly forbear expostulating with my destiny: Is this thy pastime, O Nature, to put such tricks upon a silly creature, and then to take off the mask, and show him how he hath been befooled? If this is the philosophy of human nature, my soul enter thou not into her secrets. It is surely the forbidden tree of knowledge; I no sooner taste it, than I perceive myself naked, and stript of all things - yea even of my very self. I see myself, and the whole frame of nature, shrink into fleeting ideas, which, like Epicurus's atoms, dance about in emptiness.
'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.
But man is not only a percipient being; he is also a being of will, and as such he comes into a relationship with the world which can be a source of rich experience. If one observes this relationship, one is bound to notice that it is based on the self-evident assumption that one possesses a lasting individuality, whose actions deal with a lasting material world. Any other way of behaviour would contradict the common sense of man; where we meet with it we are faced with a lunatic.
Thus philosophy and common sense seem to stand in irreconcilable opposition to each other. But this opposition is only apparent. It exists so long as philosophy thinks it is able to come to valid conclusions without listening to the voice of common sense, believing itself to be too exalted to need to do so. Philosophy, then, does not realize 'that it has no other root but the principles of Common Sense; it grows out of them, and draws its nourishment from them: severed from this root, its honours wither, its sap is dried up, it dies and rots.' (I, 5.)
At the moment when the philosophical consciousness ceases to regard itself as the sole foundation of its existence and recognizes that it can say nothing about itself without considering the source from which it has evolved, it attains the possibility of seeing the content of its experience in a new light. For it is no longer satisfied with considering this content in the completed form in which it presents itself. Rather does it feel impelled to investigate the process which gives rise to this content as an end-product (the 'impressions' and 'ideas' of Hume and his predecessors).