groundwork of their reasoning; and though, as a general principle, it has been lately called in question by a few of the ablest advocates for what they have ventured to denominate the Theory of Common Sense, and especially by Professor Stewart*, (as I may perhaps find it necessary to notice more particularly hereafter,) it is sufficient for the present to observe that the shrewd and learned projector of this theory, Dr. Reid, admits it in its utmost latitude: "Wise men," says he, "now agree or ought to agree in this, that there is but one way to the knowledge of nature's works, the way of observation and experiment. By our constitution we have a strong propensity to trace particular facts and observations to general rules, and to apply such general rules to account for other effects, or to direct us in the production of them. This procedure of the understanding is familiar to every human creature in the common affairs of life, and IT IS THE ONLY ONE BY WHICH ANY REAL DISCOVERY IN PHI LOSOPHY CAN BE MADE."† Now the only mode by which we can obtain experience is by the use and exercise of the senses, which have been given to us for this purpose, and which, to speak figuratively, may be regarded as the fingers of the mind in feeling its way forward, and opening the shutters to the admission of that pure and invigorating light, which in consequence breaks in upon it. It must be obvious, however, to every one who has attended to the operations of his senses, that * Philos. Essays, vol. i. p. 122. is there never is, nor can be, any direct communication between the mind and the external objects the mind perceives, which are usually, indeed, at some distance from the sense that gives notice of them. Thus, in looking at a tree, it is the eye alone that really beholds the tree, while the mind only receives a notice of its presence, by some means or other, from the visual organ. So in touching this table, it my hand alone that comes in contact with it, and communicates to my mind a 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 a faculty of producing, like a looking-glass, accurate countersigns, 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 St. 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.* There is scarcely, however, an hypothesis which has been started in modern times that has not 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, * See the author's Study of Medicine, vol. iv. p. 46. edit. 2. 1825. 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 enquiry 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 times 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 Plato, of Aristotle, of Epicurus, and of the sceptics, especially Pyrrho and Arcecilas; and the principal systems to which they have given birth in latter 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 eternal, is also, in its prímal and simple state, essentially amorphous, or destitute of all form and quality whatever; and I further remarked, that the groundwork of this dogma consisted in a belief that form and quality are the contrivance of an intelligent * Vol. I. Ser. 1. Lect. II. 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 age; 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 |