ΤΟ

MR. DUGALD STEWART,

LATELY PROfessor of MATHEMATICS, NOW PROFESSOR OF MORAL

PHILOSOPHY;

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 Kames 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 cotemporaries.

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.

THO. 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 knowledge or intention. But, laying aside conjecture, and all pretences to determine in things beyond our reach, we must rest in this, that body and mind are the only kinds of being of which we can have any knowledge, or can form any conception. If there be other kinds, they are not discoverable by the faculties which God hath given us; and with regard to us, are as if they were not.

As, therefore, all our knowledge is confined to body and mind, or things belonging to them, there are two great branches of philosophy, one relating to body, the other to mind. The properties of body, and the laws that obtain in the material system, are the objects of Natural Philosophy, as that word is now used. The branch which treats of the nature and operations of mind has by some been called Pneumatology. And to the one or the other of these branches, the principles of all the sciences belong.

What variety there may be of minds or thinking beings throughout this vast universe, we cannot pretend to say. We dwell in a little corner of God's dominion, disjoined from the rest of it. The globe which we inhabit is but one of seven planets that encircle our sun. What various orders of beings may inhabit the other six, their secondaries, and the comets, belonging to our system; and how many other suns may be encircled with like systems, are things altogether hid from us. Although human reason and industry have discovered with great accuracy the order and distances of the planets, and the laws of their motion, we have no means of corresponding with them. That they may be the habitation of animated beings is very probable but of the nature, or powers of their inhabitants, we are perfectly ignorant. Every man is conscious of a thinking principle or mind in himself, and we have sufficient evidence of a like principle in other men. The actions of brute animals show, that they have some thinking principle, though of a nature far inferior to the human mind. And every thing about us may convince us of the existence of a Supreme Mind, the Maker and Governor of the Universe. These are all the minds of which reason can give us any certain knowledge.

The mind of man is the noblest work of God which reason discovers to us, and therefore, on account of its dignity, deserves our study. It must indeed be acknowledged, that although it is of all objects the nearest to us, and seems the most within our reach, it is very difficult to attend to its operations, so as to form a distinct notion of them; and on that account there is no branch of knowledge in which the ingenious and speculative have fallen into so great errors, and even absurdities. These errors and absurdities have given rise to a general prejudice against all inquiries of this nature; and because ingenious men have, for many ages, given different and contradictory accounts of the powers of the mind, it is concluded, that all speculations concerning them are chimerical and visionary.

But whatever effect this prejudice may have with superficial thinkers, the judicious will not be apt to be carried away with it. About two hundred years ago, the opinions of men in natural philosophy were as various, and as contradictory, as they are now concerning the powers of the mind. Galileo, Torricelli, Kepler, Bacon, and Newton, had the same discouragement in their attempts to throw light upon the material system, as we have with regard to the intellectual. If they had been deterred by such we should never have reaped the benefit of their discoveries,

which do honour to human nature, and will make their names immortal. The motto which Lord Bacon prefixed to some of his writings was worthy of his genius, Inveniam viam aut faciam.

There is a natural order in the progress of the sciences, and good reasons may be assigned why the philosophy of body should be elder sister to that of mind, and of a quicker growth; but the last hath the principle of life no less than the first, and will grow up, though slowly, to maturity. The remains of ancient philosophy upon this subject are venerable ruins, carrying the marks of genius and industry, sufficient to inflame, but not to satisfy, our curiosity. In later ages, Des Cartes was the first that pointed out the road we ought to take in those dark regions. Malebranche, Arnauld, Locke, Berkeley, Buffier, Hutcheson, Butler, Hume, Price, Lord Kames, have laboured to make discoveries; nor have they laboured in vain. For, however different and contrary their conclusions are, however sceptical some of them, they have all given new light, and cleared the way to those who shall come after them.

We ought never to despair of human genius, but rather to hope, that, in time, it may produce a system of the powers and operations of the human mind, no less certain than those of optics or astronomy.

This is the more devoutly to be wished, that a distinct knowledge of the powers of the mind would undoubtedly give great light to many other branches of science. Mr. Hume hath justly observed, that "all the sciences have a relation to human nature; and, however wide any of them may seem to run from it, they still return back by one passage or another. This is the centre and capital of the sciences, which being once masters of, we may easily extend our conquests every where."

The faculties of our minds are the tools and engines we must use in every disquisition; and the better we understand their nature and force, the more successfully we shall be able to apply them. Mr. Locke gives this account of the occasion of his entering upon his Essay concerning Human Understanding: "Five or six friends (says he) meeting at my chamber and discoursing on a subject very remote from this, found themselves quickly at a stand, by the difficulties that rose on every side. After we had for a while puzzled ourselves, without coming any nearer to a resolution of those doubts that perplexed us, it came into my thoughts that we took a wrong course; and that, before we set ourselves upon inquiries of that nature, it was necessary to examine our own abilities, and see what objects our understandings were fitted or not fitted to deal with. This I proposed to the company, who all readily assented; and thereupon it was agreed that this should be our first Inquiry." If this be commonly the cause of perplexity in those disquisitions which have least relation to the mind, it must be so much more in those that have an immediate connexion with it.

The sciences may be distinguished into two classes, according as they pertain to the material or to the intellectual world. The various parts of