Reid, like his contemporary Kant, felt his philosophical conscience stirred by Hume's Treatise of Human Nature, and, like Kant, set himself the task of opposing it. Unlike Kant, however, whose philosophic system was designed to arrest man's reason before the abyss into which Hume threatened to cast it, Reid contrives to detect the bridge that leads safely across this abyss. Even though it was not granted to him actually to set foot on this bridge (this, in his time, only Goethe managed to do), he was able to describe it in a manner especially helpful for our own purpose.
The first of the three books in which Reid set out the results of his labours appeared in 1764 under the title, Inquiry into the Human Mind on the Principles of Common Sense. The other two, Essays on the Intellectual Powers of Man and Essays on the Active Powers of Man, appeared twenty years later. In these books Reid had in view a more all-embracing purpose than in his first work. The achievement of this purpose, however, required a greater spiritual power than was granted to him. Comparing his later with his earlier work, Reid's biographer, A. Campbell Fraser, says:
'Reid's Essays form, as it were, the inner court of the temple of which the Aberdonian Inquiry is the vestibule. But the vestibule is a more finished work of constructive skill than the inner court, for the aged architect appears at last as if embarrassed by accumulated material. The Essays, greater in bulk, perhaps less deserve a place among modern philosophical classics than the Inquiry, notwithstanding its narrower scope, confined as it is to man's perception of the extended world, as an object lesson on the method of appeal to common sense.'
Whilst the ideas of Kant, by which he tried in his way to oppose Hume's philosophy, have become within a short space of time the common possession of men's minds, it was the fate of Reid's ideas to find favour among only a restricted circle of friends. Moreover, they suffered decisive misunderstanding and distortion through the efforts of well-meaning disciples. This was because Kant's work was a late fruit of an epoch of human development which had lasted for centuries and in his time began to draw to its close, while Reid's work represents a seed of a new epoch yet to come. Here lies the reason also for his failure to develop his philosophy beyond the achievements contained in his first work. It is on the latter, therefore, that we shall chiefly draw for presenting Reid's thoughts.
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The convincing nature of Hume's argumentation, together with the absurdity of the conclusions to which it led, aroused in Reid a suspicion that the premises on which Hume's thoughts were built, and which he, in company with all his predecessors, had assumed quite uncritically, contained some fundamental error. For both as a Christian, a philosopher, and a man in possession of common sense, Reid had no doubt as to the absurdity and destructiveness of the conclusions to which Hume's reasoning had led him.
'For my own satisfaction, I entered into a serious examination of the principles upon which this sceptical system is built; and was not a little surprised to find that it leans with its whole weight upon a hypothesis, which is ancient indeed, and hath been very generally received by philosophers, but of which I could find no solid proof. The hypothesis I mean is, That nothing is perceived but what is in the mind which perceives it: That we do not really perceive the things that are external, but only certain images and pictures of them imprinted upon the mind, which are called impressions and ideas.
'If this be true, supposing certain impressions and ideas to exist presently in my mind, I cannot, from their existence, infer the existence of anything else; my impressions and ideas are the only existences of which I can have any knowledge or conception; and they are such fleeting and transitory beings, that they can have no existence at all, any longer than I am conscious of them. So that, upon this hypothesis, the whole universe about me, bodies and spirits, sun, moon, stars, and earth, friends and relations, all things without exception, which I imagined to have a permanent existence whether I thought of them or not vanish at once:
'And, like the baseless fabric of this vision ... Leave not a rack behind.
'I thought it unreasonable, upon the authority of philosophers, to admit a hypothesis which, in my opinion, overturns all philosophy, all religion and virtue, and all common sense: and finding, that all the systems which I was acquainted with, were built upon this hypothesis, I resolved to enquire into this subject anew, without regard to any hypothesis.'