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knowledge of its hardness and other qualities. What then is the medium by which such communication is maintained which induces the mind, seated as it is in some undeveloped part of the brain, to have a correspondent perception of the form, size, colour, smell, and even distance of objects with the senses which are seated on the surface of the body: and which, at the same time that it conveys this information, produces such an additional effect that the mind is able at its option to revive the perception, or call up an exact notion or idea of these qualities at a distant period, or when the objects themselves are no longer present? Is there, or is there not, any resemblance between the external or sensible object and the internal or mental idea or notion? If there be a resemblance, in what does that resemblance consist? and how is it produced and supported? Does the external object throw off representative likenesses of itself in films, or under any other modification, so fine as to be able, like the electric or magnetic aura, to pass without injury from the object to the sentient organ, and from the sentient organ to the sensory? Or has the mind itself the faculty of producing, like a looking-glass, accurate counter-signs, intellectual pictures or images, correspondent with the sensible images communicated from the external object to the sentient organ? If, on the contrary, there be no resemblance, are the mental perceptions mere notions or intellectual symbols excited in it by the action of the external sense; which, while they bear no similitude to the qualities of the object discerned, answer the purpose of those qualities, as letters answer the purpose of sounds? Or are we sure that there is any external world whatever any thing beyond the intellectual principle that perceives, and the sensations and notions that are perceived; or even any thing beyond those sensations and notions, those impressions and ideas themselves?

Several of these questions may perhaps appear in no small degree whimsical and brain-sick, and more worthy of Saint Luke's than of a scientific institution. But all of them, and perhaps as many more, of a temperament as wild as the wildest, have been asked, and insisted upon, and supported again and again in different ages and countries, by philosophers of the clearest intellects in other respects, and who had no idea of labouring under any such mental infirmity, nor ever dreamed of the necessity of being blistered and taking physic.*

There is scarcely, however, a hypothesis which has been started in modern times that cannot look for its prototype or suggestion among the ancients and it will hence be found most advantageous, and may perhaps prove the shortest way, to begin at the fountain-head, and to trace the different currents which have flowed from it. That fountain-head is Greece, or at least we may so regard it on the present occasion: and the plan which I shall request leave to pursue in the general inquiry before us will be, first of all to take a rapid sketch of the most celebrated speculations upon this subject to which this well-spring of wisdom has given rise; next, to follow up the chief ramifications which have issued from them in later periods; and lastly to summon, as by a quo warranto, the more prominent of those of our own day to appear personally before the bar of this enlightened tribunal, for the purpose 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

* See the Author's Study of Medieine, vol. iv. p. 46. Edit. 2. 1825.

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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.

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 eternal, is also, in its primal and simple state, essentially amorphous, or destitute of all form and quality whatever; and I further remarked, that the ground-work of this dogma consisted in a belief that form and quality are the contrivance 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 principally 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 embodied, 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, however, 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 insubstantial 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

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external senses. This insubstantial 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 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 dw, (idola) or SPECIES, in contradistinction to the insubstantial 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 further 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 forms and other qualities of objects have been taken; according to Aristotle, insubstantial pictures of them, as though reflected from a mirror; and, according to Epicurus, substantial or material effigies: such perceptions being under the first view of them denominated IDEAS; under the second, PHANTASMS; under the third, idola, or

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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 an universal skepticism. Of this delirious class of disputants, who were suffered to wander at large without a strait waistcoat, there are two that are preeminently entitled to our attention, 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 mo

ment-the external world becomes a mere show-and there is no truth or solidity in any thing. He was not able 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; believing 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, an excellent and accomplished scholar, that Lucretius directed thosé 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
Of 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
Of truth and falsehood, from the senses drawn,
Nor aught can e'er refute them; for what once,
By truths oppos'd, their falsehood can detect,
Must claim a trust far ampler than themselves.
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 triumphant 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, appropriate 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, 'twere 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.*

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 threw 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

Denique, nihil 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 leisure.

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