‘By Dr. Blair’s means I have been favoured with the perusal of your performance, which I have read with great pleasure and attention. It is certainly very rare that a piece so deeply philosophical is wrote with so much spirit, and affords so much entertainment to the reader, though I must still regret the disadvantages under which I read it, as I never had the whole performance at once before me, and could not be able to compare one part with another. To this reason chiefly I ascribe some obscurities which, in spite of your short analysis and abstract, seem to hang over your system; for I must do you the justice to own that when I enter into your ideas, no man appears to express himself with greater perspicuity than you do, a talent which above all others is requisite in that species of literature which you have cultivated. There are some objections which I would willingly propose to the chapter “Of Sight,” did I not suspect that they arise from my not sufficiently understanding it. I shall therefore forbear till the whole can lie before me, and shall not at present propose any further difficulties to your reasonings. I shall only say that if you have been able to clear up these abstruse and important subjects, instead of being mortified, I shall be so vain as to pretend to a share of the praise; and shall think that my errors, by having at least some coherence, have led you to make a more strict review of my principles, which were the common ones, and to perceive their futility.’
In his reply to this letter Reid recognises the debt which philosophy owes to the sceptical criticism of Hume. He writes thus to Hume:—
‘King’s College, 18th March 1763.
‘Sir,—On Monday last Mr. John Farquhar brought me your letter of February 25th, enclosed in one from Dr. Blair. I thought myself very happy in having the means of obtaining, at second hand, through the friendship of Dr. Blair, your opinion of my performance; and you have been pleased to communicate it directly in so polite and friendly a manner as merits great acknowledgments on my part.
‘In attempting to throw some light on these abstruse subjects, I wish to preserve the due mean between confidence and despair. But whether I have any success in this attempt or not, I shall always avow myself your disciple in metaphysics. I have learned more from your writings in this kind than from all others put together. Your system appears to me not only coherent in all its parts, but likewise justly deduced from principles commonly received among philosophers,—principles which I never thought of calling in question, until the conclusions you draw from them in the Treatise of Human Nature made me suspect them. If these principles are solid your system must stand; and whether they are or not can better be judged after you have brought to light the whole system that grows out of them, than when the greater part of it was wrapped up in clouds and darkness. I agree with you therefore, that if this system shall ever be demolished, you have a just claim to a great share of the praise, both because you have made it a distinct and determinate mark to be aimed at, and have furnished proper artillery for the purpose. When you have seen the whole of my performance, I shall take it as a very great favour to have your opinion upon it, from which I make no doubt of receiving light, whether I receive conviction or no.’
Hume’s philosophy being the suicide of intelligence, can hardly be spoken of as a ‘system.’ It is not constructive, but wholly destructive, and therefore a negation of system, philosophic or other, in a finally speechless and motionless agnostic doubt.
Hume’s letter to Reid was his only rejoinder to the Inquiry. In 1763 he had ceased to produce philosophical books. His recognition of the ‘deeply philosophical’ nature of Reid’s argument is a testimony to the worth of the work on which Reid’s literary reputation chiefly rests. The ‘share of the praise’ which Hume claims, Reid was ready to concede. ‘I hope,’ he says in the Dedication of the Inquiry, ‘I hope that the author of this sceptical system wrote it in the belief that it would be useful to mankind. And perhaps it may prove so at last. For I conceive the sceptical writers to be a set of men whose business it is to pick holes in the fabric of knowledge wherever it is weak and faulty; and when those places are properly repaired, the whole building becomes more firm and solid than it was formerly’. And, in fact, the intellectual progress of mankind has been sustained by an alternation of sceptical and conservative philosophies.
After prolonged cogitation, the ‘weak and faulty’ place in the ‘fabric’ of modern philosophy seemed to Reid to lie in an unproved assumption with which modern philosophers set out in their speculations. For they took for granted that only the sensations and ideas within each mind could be perceived: yet we naturally ‘believe that we are seeing and handling the very outward things themselves’ that make what is called the material world. If the philosophers are right, we have no immediate revelation of the qualities of outward things, and no evidence of their existence, except the succession of changing sensations and ideas of sensations which alone are presented to us. I can never directly encounter an external object. What I suppose to be outward is all in my mind. With a world thus imprisoned within me, is there any legitimate way through which I can assure myself of realities outside my mind, including other living persons who have their own sensations and ideas as much as I have mine, but numerically different from my private stock? If not, grave consequences seemed to follow: faith and hope must die out of human life. So Reid argued.
The Treatise of Human Nature was the scarecrow that warned Reid of the destructive consequences of this assumption, that we are shut up among our own ideas; as he had himself in youth been taught to believe, and by which he had been led on to Berkeley’s conception of a wholly ideal material world. He now began to see this ideal material world of Berkeley in a new light. It seemed to leave him alone in the universe of his own mind. The ideal matter of Berkeley cannot, he argued, inform me of the existence of other living persons: what I have supposed to be other persons must be, like the rest, only some of my own ideas. If I allow that my own sensations and ideas are my only possible original data, I cannot from such transitory phantoms infer the real existence of other persons. With this starting-point as my only one, the whole universe naturally supposed to surround me—bodies and spirits, friends and relations, all dead things and all living persons—acknowledged by common judgment to have a permanent existence, whether I am having ideas of them or not—vanish at once. My private sensations and ideas can never really signify to me the existence of other conscious beings; only what might be called public ideas, common to other minds and to me, could do this. And such objects are not properly called ‘ideas’—public or private; they are dead things and living persons; they exist whether I individually have sensations and ideas or not. In short, Reid could find nothing in Berkeley’s philosophical theory—as he interpreted it[11]—which afforded even probable ground for concluding that there were in existence other intelligent beings—fathers, brothers, friends, or fellow-citizens. ‘I am left alone,’ the only creature of God, in that forlorn state of egoism into which it was said some of the disciples of Descartes were brought by this same preliminary assumption of their master—a sufficient reductio ad absurdum of the favourite postulate which leads to it. To act upon it would argue insanity in the agent; it must therefore be empty verbal speculation. So Reid further argued.