DEDICATION. то MR. DUGALD STEWART, LATELY PROFESSOR OF MATHEMATICS, NOW PROFESSOR AND DR. JAMES GREGORY, PROFESSOR OF THE THEORY OF PHYSIC, IN THE UNIVERSITY OF EDINBURGH. My dear Friends, I KNOW not to whom I can address these Essays with more propriety than to you; not only on account of a friendship begun in early life on your part, though in old age on mine, and in one of you I may say hereditary; nor yet on account of that correspondence in our literary pursuits and amusements, which has always given me so great pleasure; but because, if these Essays have any merit, you have a considerable share in it, having not only encouraged me to hope that they may be useful, but favoured me with your observations on every part of them, both before they were sent to press and while they were under it. I have availed myself of your observations, so as to correct many faults that might otherwise have escaped me; and I have a very grateful sense of your friendship, in giving this aid to one, who stood much in need of it; having no shame, but much pleasure, in being instructed by those who formerly were my pupils, as one of you was. It would be ingratitude to a man whose memory I most highly respect, not to mention my obligations to the late lord Kaimes for the concern he was pleased to take in this work. Having seen a small part of it, he urged me to carry it on; took account of my progress from time to time; revised it more than once, as far as it was carried, before his death; and gave me his observations on it, both with respect to the matter and the expression. On some points we differed in opinion, and debated them keenly, both in conversation and by many letters, without any abatement of his affection, or of his zeal for the work's being carried on and published: for he had too much liberality of mind not to allow to others the same liberty in judging which he claimed to himself. It is difficult to say whether that worthy man was more eminent in active life or in speculation. Very rare, surely, have been the instances where the talents for both were united in so eminent a degree. His genius and industry, in many different branches of literature, will, by his works, be known to posterity. His private virtues, and public spirit, his assiduity, through a long and laborious life, in many honourable public offices with which he was intrusted, and his zeal to encourage and promote every thing that tended to the improvement of his country, in laws, literature, commerce, manufactures, and agriculture, are best known to his friends and contemporaries. The favourable opinion which he, and you, my friends, were pleased to express of this work, has been my chief encouragement to lay it before the public; and perhaps, without that encouragement, it had never seen the light: for I have always found, that, without social intercourse, even a favourite speculation languishes; and that we cannot help thinking the better of our own opinions when they are approved by those whom we esteem good judges. You know that the substance of these Essays was delivered annually, for more than twenty years, in lectures to a large body of the more advanced students in this university, and for several years before, in another university. Those who heard me with attention, of whom I presume there are some hundreds alive, will recognize the doctrine which they heard, some of them thirty years ago, delivered to them more diffusely, and with the repetitions and illustrations proper for such audiences. I am afraid, indeed, that the more intelligent reader, who is conversant in such abstract subjects, may think that there are repetitions still left, which might be spared. Such, I hope, will consider, that what to one reader is a superfluous repetition, to the greater part, less conversant in such subjects, may be very useful. If this apology be deemed insufficient, and be thought to be the dictate of laziness, I claim some indulgence even for that laziness, at my period of life. You who are in the prime of life, with the vigour which it inspires, will, I hope, make more happy advances in this, or in any other branch of science to which your talents may be applied. GLASGOW COLLEGE, June 1, 1785. THOMAS REID. PREFACE. HUMAN knowledge may be reduced to two general heads, according as it relates to body or to mind; to things material, or to things intellectual. The whole system of bodies in the universe, of which we know but a very small part, may be, called the Material World; the whole system of minds, from the infinite Creator, to the meanest creature endowed with thought, may be called the Intellectual World. These are the two great kingdoms of nature that fall within our notice; and about the one, or the other, or things pertaining to them, every art, every science, and every human thought is employed; nor can the boldest flight of imagination carry us beyond their limits. Many things there are, indeed, regarding the nature and the structure both of body and of mind, which our faculties cannot reach; many difficulties which the ablest philosopher cannot resolve; but of other natures, if any other there be, we have no knowledge, no conception at all. That every thing that exists must be either corporeal or incorporeal, is evident. But it is not so evident, that every thing that exists must either be corporeal, or endowed with thought. Whether there be in the universe, beings which are neither extended, solid, and inert, like body, nor active and intelligent, like mind, seems to be beyond the reach of our knowledge. There appears to be a vast interval between body and mind, and whether there be any intermediate nature that connects them together, we know not. We have no reason to ascribe intelligence, or even sensation, to plants; yet there appears in them an active force and energy, which cannot be the result of any arrangement or combination of inert matter. The same thing may be said of those powers by which animals are nourished and grow, by which matter gravitates, by which magnetical and electrical bodies attract and repel each other, and by which the parts of solid bodies cohere. Some have conjectured, that the phenomena of the material world which require active force, are produced by the continual operation of intelligent beings: others have conjectured, that there may be in the universe, beings that are active without intelligence, which, as a kind of incorporeal machinery, contrived by the Supreme Wisdom, perform their destined task without any |