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pose of trying their comparative pretensions, and of submitting them to your impartial award. ,

The principal systems that were started among the philosophers of Greece to explain the origin and value of human knowledge were those of Plato, of Aristotle, of Epicurus, and of the skeptics, especially Pyrrho and Arcesilas; and the principal systems to which they have given birth in later or modern times, are those of Des Cartes, Locke, Berkeley, Hume, Hartley, Kant, and the Scottish School of Common Sense, at the head of which we are to place Dr. Reid.

I had occasion to observe, in our first series of lectures,* that it was a dogma common to many of the Greek schools, that matter, though essentially eter nal, is also, in its primal and simple state, essentially amorphous, or desti. tute of all form and quality whatever; and I farther remarked, that the groundwork of this dogma consisted in a belief that form and quality are the cons trivance of an intelligent agent; while matter, though essentially eternal, is essentially unintelligent. Matter, therefore, it was contended, cannot possibly assume one mode of form rather than another mode; for if it were capable of assuming any kind, it must have been capable of assuming every kind. and of course of exhibiting intelligent effects without an intelligent cause.

Form, then, according to the Platonic schools, in which this was princi. pally taught, existing distinct from matter by the mere will of the Great First Cause, presented itself, from all eternity, to his wisdom or logos, in every possible variety; or, in other words, under an infinite multiplicity of incorporeal or intellectual patterns, exemplars, or archetypes, to which the founder of this school gave the name of ideas; a term that has descended without any mischief into the popular language of our own day; but which, in the hands of the schoolmen, and various other theorists, has not unfrequently been productive of egregious errors and abuses. By the union of these intellectual archetypes with the whole or with any portion of primary or incorporeal matter, matter immediately becomes imbodied, assumes palpable forms, correspondent with the archetypes united with it, and is rendered an object of perception to the external senses; the mind, or intelligent principle itself, how. ever, which is an emanation from the Great Intelligent Cause, never perceiving any thing more than the intellectual or formative ideas of objects as they are presented to the senses, and reasoning concerning them by those ideas alone.

It must be obvious, however, that the mind is possessed of many ideas which it could not derive from a material source. Such are all those that relate to abstract moral truths and pure mathematics. And to account for these, it was a doctrine of the Platonic philosophy, that, besides the sensible world, there is also an intelligible world; that the mind of man is equally connected with both, though the latter cannot possibly be discerned by corporeal organs; and that, as the mind perceives and reasons upon sensible objects by means of sensible archetypes or ideas, so it perceives and reasons upon intelligible objects by means of intelligible ideas.

The only essential variation from this hypothesis which Aristotle appears to have introduced into his own, consists in his having clothed, if I may be allowed the expression, the naked ideas of Plato, with the actual qualities of the objects perceived ; his doctrine being, that the sense, on perceiving or being excited by an external object, conveys to the mind a real resemblance of it; which, however, though possessing form, colour, and other qualities of matter, is not matter itself, but an unsubstantial image, like the picture in a mirror; as though the mind itself were a kind of mirror, and had a power of reflecting the image of whatever object is presented to the external senses. This unsubstantial image or picture, in order to distinguish it from the intellectual pattern or idea of Plato, he denominated a phantasm. And as he supported with Plato the existence of an intelligible as well as of a sensible world, it was another part of his hypothesis that, while things sensible are

* Serres i Lecture ii.

perceived by sensible phantasms, things intelligible are perceived by intelligible phantasms; and consequently that virtue and vice, truth and falsehood, time, space, and numbers, have all their pictures and phantasms, as well as plants, houses, and animals.

Epicurus admitted a part of this hypothesis, and taught it contemporaneously at Mitylene, but the greater part he openly opposed and ridiculed. He concurred in the doctrine that the mind perceives sensible objects by means of sensible images; but he contended that those images are as strictly material as the objects from which they emanate; and that if we allow them to possess material qualities, we must necessarily allow them at the same time to possess the substance to which such qualities appertain. Epicurus, therefore, believed the perceptions of the mind to be real and substantial effigies, and to these effigies he gave the name of ciowda (idola), or SPECIES, in contradistinction to the unsubstantial PHANTASMS of Aristotle, and the intellectual or formative iDEAS of Plato. He maintained that all external objects are perpetually throwing off fine alternate waves of different flavours, odours, colours, shapes, and other qualities; which, by striking against their appropriate senses, excite in the senses themselves a perception of the qualities and presence of the parent object; and are immediately conveyed by the sentient channel to the chamber of the mind, or sensory, without any injury to their texture : in the same manner as heat, light, and magnetism pervade solid substances, and still retain their integrity. And he affirmed, farther, that instead of the existence of an imaginary intelligible world, throwing off intelligible images, it is from the sensible or material world alone that the mind, by the exercise of its proper faculties, in union with that of the corporeal senses, derives every branch of knowledge, physical, moral, or mathematical.

If this view of the abstruse subject before us be correct, as I flatter myself it is, I may recapitulate in few words, that the external perceptions of the mind are, according to Plato, the primitive or intellectual patterns from which the forins and other qualities of objects have been taken ; according to Aris. totle, unsubstantial pictures of them, as though reflected from a mirror; and, according to Epicurus, substantial or material effigies ; such perceptions be ing under the first view of them denominated IDEAS; under the second, PHANTASMS; under the third, idola, or SPECIES.

While such were the fixed and promulgated tenets of Plato, Aristotle, and Epicurus, there were other philosophers of Greece, or who at least have been so denominated, that openly professed themselves to be without tenets of any kind; who declared that nothing was known or could be known upon any subject; and who, consequently, inculcated a universal skepticism. Of this delirious class of disputants, who were suffered to wander at large without a strait waistcoat, there are two that are pre-eminently entitled to our atten. tion, Pyrrho and Arcesilas. Pyrrho studied first in the atomic school of Democritus, and seems to have lost his senses upon the question of the infinite divisibility of matter, a question which has not unfrequently given birth to the same disease in modern times. He first doubted the solidity of its elementary atoms,--he next found out, that if these be not solid, every thing slips away from the fingers in a moment--the external world becomes a mere show -and there is no truth or solidity in any thing. He was not abie to prove the solidity of the elementary atoms of matter. He hence doubted of every thing; advised all the world to do the same; and established a school for the purpose of inculcating this strange doctrine. In every other respect he was a man of distinguished accomplishments, and so highly esteemed by his countrymen, as to have been honoured with the dignity of chief priest, and exempted from public taxation. But to such a formidable extreme did this disease of skepticism carry him, that one or more of his friends, as we are gravely told in history, were obliged to accompany him wherever he went, that he might not be run over by carriages, or fall down precipices. Yet he contrived, by some means or other, to live longer than most men of caution and common sense ; for we find him at last dying of a natural death, at the good old age of ninety.

Arcesilas was one of the successors to Plato in the academic chair, and founder of the school that has been known by the name of the MIDDLE ACADEMY. Plato, in his fondness for intellectual IDEAS, those creatures of his own imagination, had always given a much greater degree of credit to their testimony than to that of the objects which compose the material world ; be. lieving that the mind was less likely to be imposed upon than the external senses. And with so much zeal was this feeling or prejudice followed up by Arcesilas, that he soon began to doubt, and advised his scholars to doubt also, of the reality of every thing they saw about them; and at length terminated his doubts in questioning the competency of reason itself to decide upon any evidence the external senses might produce, though he admitted an external world of some kind or other. And upon being reminded, by one of his scholars, who had a wish to please him, that the only thing which Socrates declared he was certain of was his own ignorance, he immediately replied, that Socrates had no right to say even that-for that no man could be certain of any thing. It was against this unhappy madman, though, in other respects, like Pyrrho, excellent and accomplished scholar, that Lucretius directed those forcible verses in favour of the truth and testimony of the senses, as the only genuine means of acquiring knowledge, which have been so often referred to, and so warmly commended in the controversy of the present day :

Who holds that naught is known, denies he knows
E'en this, thus owning that he nothing knows.
With such I ne'er could reason, who, with face
Retorted, treads the ground just trod before.

Yet grant e'en this he knows ; since naught exists
Or truth in things, whence learns he what to know,
Or what not know! What things can give him first
The notion crude of what is false or true?
What prove aught doubtful, or of doubt devoid !

Search, and this earliest notion thou wilt find
or truth and falsehood, from the senses drawn,
Nor aught can e'er refute them ; for what once,
By truths oppos'd, their faisehood can detect,
Must claim a trust far ampler than themselvon,
Yet what, than these, an ampler trust can claim
Can reason, born, forsooth, of erring sense,
Impeach those senses whence alone it springs ?
And which, if false, itself can ne'er be true.
Can sight correct the ears? Can ears the touch!
Or touch the tongue's fine flavour? or, o'er all
Can smell triunphant rise? Absurd the thought !
For every sense a separate function boasts,
A power prescrib'd: and hence, or soft, or hard,
Or hot, or cold, to its appropriate sense
Alone appeals. The gaudy train of hues,
With their light shades, appropriute thus, alike
Perceive we; tastes appropriate powers possess;
Appropriate sounds and odours ; and hence, too,
One sense another ne'er can contravene,
Nor e'en correct itself; since, every hour,
In every act, each claims an equal faith.

E'en though the mind no real cause could urge
Why what is square when present, when remote
Cylindric seems, 't were dangerous less to adopt
A cause unsound, than rashly yield at once
All that we grasp of truth and surety most;
Rend all reliance, and root up, forlorn,
The first firm principles of life and health.
For not alone fails reason, life itself
Ends instant, if the senses thou distrust,
And dare some dangerous precipice, or aught
Against warn'd equal, spurning what is safe.
Hence all against the senses urg'd is vain;
Mere idle rant, and hollow pomp of words.

As, in a building, if the first lines err,
If aught impede the plummet, or the rule
From its just angles deviate but a hair,
The total edifice must rise untrue,
Recumbent, curv'd, o'erhanging, void of grace,
Tumbling or tumbled from this first defect,
So must all reason prove unsound, deduc'd
From things created, if the senses err.*

* Denique, rihil sciri si quis putat, id quoque nescit

An sciri possit, &c.-Lib. iv. 471.
The passage is too long for quotation, and the reader may easily turn to it at his leisan.

It is not to be supposed that mankind could consent to be inoculated with this disease to any great extent, or for any considerable period of time : and hence the chief hypotheses that were countenanced at Rome, and till the decline of the Roman empire, were those of Plato, Aristotle, and Epicurus. During the dark ages, Aristotle seems to have held an undivided sovereignty : and though his competitors came in for a share of power upon the revival of literature, he still held possession of the majority of the schools, till, in the middle of the seventeenth century, Des Cartes introduced a new hypothesis, which served as a foundation for most of the systems or speculations which have appeared since.

With Aristotle and Epicurus Des Cartes contended that the mind perceives external objects by images or resemblances presented to it: these images he called, after Plato, ideas; though he neither acceded to the meaning of this term as given by Plato, nor allowed with Aristotle or Epicurus that they proceed from the objects themselves, and are transmitted to the mind through the channel of the senses; so that the precise signification he attached to this term is not clear. With Epicurus he ihrew away the doctrine of an intellectual world; but contended, in order to supply its place, that the mind has a large stock of ideas of its own, implanted by the hand of nature, and not derived from the world around us : ideas, therefore, that are strictly innate, and may be found on being searched for, though otherwise not necessarily present to the mind's contemplation. Among these the principal are, the idea of thought, or consciousness, of God, and of matter; all which may be fully depended upon as so many established truths: and hence, upon his hypothesis, all real knowledge flows from an internal source, or, in other words, from the mind itself. These ideas can never deceive us, though the senses may do so in their report concerning external objects; and, consequently, such ideas are chiefly to be trusted to and reasoned from even in questions that relate to the senses.

In analyzing the idea of THOUGHT, the mind, according to Aristotle, discovers it to be a power that has neither extension, figure, local motion, nor any other property commonly ascribed to body. In analyzing the idea of God, the mind finds presented to it a being necessarily and eternally existing, supremely intelligent, powerful, and perfect, the fountain of all goodness and truth, and the creator of the universe. In analyzing the idea of MATTER, the mind perceives it to be a substance possessing no other property than extent:or, in other words, as having nothing else belonging to it than length, breadth, and thickness; that space, possessing equally this property, is a part of matter, and consequently that matter is universal, and there is no vacuum. From these, and other innate ideas, compared and combined with the ideas of sensation, or those furnished to the mind by the senses, flows, on the hypothesis of Des Cartes, the whole fund of human understanding, or all the knowledge that mankind are or can be possessed of.

There are two fundamental errors, and errors, moreover, of an opposite character, that accompany, or rather introduce, this hypothesis, and to which, popular as it was at one time, it has at length completely fallen a sacrifice: These are the attempting to prove what ought to be taken for granted, and the taking for granted what ought to be proved.

The philosophy of Des Cartes sets off with supposing that every man is more or less under the influence of prejudice, and consequently that he cannot know the real truth of any thing till he has thoroughly sifted it. It follows, necessarily, as a second position, that every man ought, at least once in his life, to doubt of every thing, in order to sist it; not, however, like the skeptics of Greece, that, by such examination, he may be confirmed in doubt, but that, by obtaining proofs, he may have a settled conviction.

Full fraught with these preliminary principles, our philosopher opens his career of knowledge, and while he himself continues as grave as the noble knight of La Mancha, his journey commences almost as ludicrously. His first doubt is, whether he himself is alive or in being, and his next, whether any body is alive or in being about him. He soon satisfies himself, however,

upon the first point, by luckily finding out that he thinks, and, therefore, says he gravely, I must be alive: Cogito, ergo sum. “ I think, and therefore I am." And he almost as soon satisfies himself upon the second, by feeling with his hands about him, and finding out that he can run them against a something or a somebody else, against a man or a post. He then returns home to himself once more, overjoyed with this demonstration of his fingers ; and commences a second voyage of discovery by doubting whether he knows any thing besides his own existence, and that of a something beyond him. And he now ascertains, to his inexpressible satisfaction, that the soil of his own mind is sown with indigenous ideas precisely like that of thought or consciousness. These he digs up one after another, in order to examine them. One of the first that turns up is that of a God: one of the next is an idea that informs him that the outside of himself, or rather of his mind, is matter; and combining the whole he has thus far acquired with other information obtained from the same sources, he finds that the people whom he has before discovered by means of his hands and eyes call this matter a body, and that the said people have bodies of the same kind, and also the same kind of knowledge as himself, although not to the same extent or demonstration; and for this obvious reason, because they have not equally doubted and examined.

It is difficult to be grave upon such a subject. What would be thought or said of any individual in the present audience, who should rise up and openly tell us that he had been long troubled with doubts whether he really existed or not; that his friends had told him he did, and he was inclined to believe so; but that as this belief might be a mere prejudice, he was at length determined to try the fact by asking himself this plain question,—“ Do I think?" Is there a person before me but would exclaim, almost instinctively, “Ah! poor creature, he had better ask himself another plain question, whether he is in his sober senses?"

If, however, we attempt to examine seriously the mode which M. Des Cartes thus proposes of following up his own principles, it is impossible not to be astonished at his departure from them at the first outset. Instead of doubting of every thing and proving every thing, the very first position before him he takes for granted : “ I think, therefore I am." of these two positions, he makes the first the proof of the second, but what is the proof of the first! If it be necessary to prove that he is, the very groundwork of his system renders it equally necessary to prove that he thinks. But this he does not attempt to do: in direct contradiction to his fundamental principles he here commits a petitio principii, and takes it for granted. I do not find fault with him for taking it for granted; but then he might as well have saved himself the trouble of manufacturing an imperfect syllogism, and have taken it for granted also that he was alive or that he existed, for the last fact must have been just as obvious to himself as the first, and somewhat more so to the world at large.

There is another logical error in this memorable enthymeme, or syllogism without a head, which ought not to pass without notice; I mean, that the proof does not run parallel with the predicate, and, consequently, does not answer its purpose. The subject predicated is, that the philosopher exists or is alive, and to prove this he atfirms gratuitously that he thinks. “I think, and therefore I am.” Now, in respect to the extent or parallelism of the proof, he might just as well have said “ I itch,” or “ I eat, and therefore I am." I will not dispute that in all probability he thought more than he itched, or partook of food : but let us take which proof we will, it could only be a proof so long as he itched, or was eating; and, consequently, whenever he ceased from either of these conditions, upon his own argument, he would have no proof whatever of being alive. Now, that he must often have ceased from itching, or eating, there is no difficulty in admitting ; but then he may also at times have ceased from thinking, not only in various morbid states of the brain, but whenever he slept without dreaming. And hence, the utmost that any such argument could decide in his favour, let us take which kind of proof we will, would be, that he could alternately prove himself to be alive and alter

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