matter are the least grossly and palpably exposed to our observation. To the former of these circumstances is to be ascribed the general principle upon which all the known theories of perception proceed; that, in order to explain the intercourse between the mind and distant objects, it is necessary to suppose the existence of something intermediate, by which its perceptions are produced; to the latter, the various metaphorical expressions of ideas, species, forms, shadows, phantasms, images, which, while they amused the fancy with some remote analogies to the objects of our senses, did not directly revolt our reason, by presenting to us any of the tangible qualities of body.

"It was the doctrine of Aristotle," says Dr. Reid," that as our senses cannot receive external material objects themselves, they receive their species; that is, their images or forms without the matter, as wax receives the form of the seal without any of the matter of it. These images or forms, impressed upon the senses, are called sensible species, and are the objects only of the sensitive part of the mind: but by various internal powers, they are retained, refined, and spiritualized, so as to become objects of memory and imagination; and, at last, of pure intellection. When they are objects of memory and of imagination, they get the name of phantasms. When, by farther refinement, and being stripped of their particularities, they become objects of science, they are called intelligible species; so that every immediate object, whether of sense, of memory, of imagination, or of reasoning, must be some phantasm, or species, in the mind itself.

"The followers of Aristotle, especially the schoolmen, made great additions to this theory, which the author himself mentions very briefly, and with an appearance of reserve. They entered into large disquisitions with regard to the sensible species, what kind of things they are; how they are sent forth by the object, and enter by the organs of the senses; how they are preserved and refined by various agents, called internal senses, concerning the number and offices of which they had many controversies."1

The Platonists, too, although they denied the great doctrine of the Peripatetics, that all the objects of human understanding enter at first by the senses, and maintained that there exist eternal and immutable ideas, which were prior to the objects of sense, and about which all science was employed; yet appear to have agreed with them in their notions concerning the mode in which external objects are perceived. This Dr. Reid infers, partly from the silence of Aristotle about any difference between himself and his master upon this point, and partly from a passage in the seventh book of Plato's Republic, in which he compares the process of the mind, in perception, to that of a person in a cave, who sees not external objects themselves, but only their shadows.1

"Two thousand years after Plato, (continues Dr. Reid,) Mr. Locke, who studied the operations of the human mind so much, and with so great success, represents our manner of perceiving external objects by a similitude very much resembling that of the cave. Methinks,' says he, the understanding is not much unlike a closet wholly shut from light, with only some little opening left, to let in external visible resemblances or ideas of things without. Would the pictures coming into such a dark room but stay there, and lie so orderly as to be found upon occasion, it would very much resemble the understanding of a man, in reference to all objects of sight, and the ideas of them."2

"Plato's subterranean cave, and Mr. Locke's dark closet, may be applied with ease to all the systems of perceptions that have been invented; for they all suppose that we perceive not external objects immediately, and that the immediate objects of perception are only certain shadows of the external objects. Those shadows or images which we immediately perceive, were by the ancients called species, forms, phantasms. Since the time of Descartes, they have commonly been called ideas; and by Mr. Hume impressions. But all philosophers, from Plato to Mr. Hume, agree in this, that we do not perceive external objects

Essays on the Intellectual Powers

of Man, p. 99.

3 See Note B.

2 Locke, on Human Understanding, book ii. chap. xi. ? 17.

immediately, and that the immediate object of perception must be some image present to the mind." On the whole, Dr. Reid remarks, "that in their sentiments concerning perception, there appears an uniformity which rarely occurs upon subjects of so abstruse a nature."1

The very short and imperfect view we have now taken of the common theories of perception, is almost sufficient, without any commentary, to establish the truth of the two general observations formerly made; for they all evidently proceed on a supposition, suggested by the phenomena of physics, that there must of necessity exist some medium of communication between the objects of perception and the percipient mind; and they all indicate a secret conviction in their authors, of the essential distinction between mind and matter; which, although not rendered by reflection sufficiently precise and satisfactory to shew them the absurdity of attempting to explain the mode of their communication, had yet such a degree of influence on their speculations, as to induce them to exhibit their supposed medium under as mysterious and ambiguous a form as possible, in order that it might remain doubtful to which of the two predicaments, of body or mind, they meant that it should be referred. By refining away the grosser qualities of matter, and by allusions to some of the most aërial and magical appearances it assumes, they endeavoured, as it were, to spiritualize the nature of their medium; while, at the same time, all their language concerning it implied such a reference to matter as was necessary for furnishing a plausible foundation for applying to it the received maxims of natural philosophy.

Another observation, too, which was formerly hinted at, is confirmed by the same historical review,-that, in the order of inquiry, the phenomena of vision had first engaged the attention of philosophers, and had suggested to them the greater part of their language with respect to perception in general; and that, in consequence of this circumstance, the common modes of expression on the subject, unphilosophical and fanciful at best, even when applied to the sense of seeing, are, in the case of all 1 Reid, pp. 116, 117.

the other senses, obviously unintelligible and self-contradictory. "As to objects of sight," says Dr. Reid, "I understand what is meant by an image of their figure in the brain; but how shall we conceive an image of their colour, where there is absolute darkness? And as to all other objects of sense, except figure and colour, I am unable to conceive what is meant by an image of them. Let any man say what he means by an image of heat and cold, an image of hardness or softness, an image of sound, or smell, or taste. The word image, when applied to these objects of sense, has absolutely no meaning." This palpable imperfection in the ideal theory, has plainly taken rise from the natural order in which the phenomena of perception present themselves to the curiosity.

The mistakes which have been so long current in the world. about this part of the human constitution, will, I hope, justify me for prosecuting the subject a little farther; in particular, for illustrating, at some length, the first of the two general remarks already referred to. This speculation I enter upon the more willingly, that it affords me an opportunity of stating some important principles with respect to the object and the limits of philosophical inquiry, to which I shall frequently have occasion to refer in the course of the following disquisitions.

SECT. II.-OF CERTAIN NATURAL PREJUDICES, WHICH SEEM TO HAVE GIVEN RISE TO THE COMMON THEORIES OF PERCEPTION.

It seems now to be pretty generally agreed among philosophers, that there is no instance in which we are able to perceive a necessary connexion between two successive events, or to comprehend in what manner the one proceeds from the other as its cause. From experience, indeed, we learn, that there are many events which are constantly conjoined, so that the one invariably follows the other: but it is possible, for any thing we know to the contrary, that this connexion, though a constant one as far as our observation has reached, may not be a necessary connexion; nay, it is possible that there may be no necessary connexions among any of the phenomena we see:

and, if there are any such connexions existing, we may rest assured that we shall never be able to discover them.1

I shall endeavour to shew, in another part of this work, that the doctrine I have now stated does not lead to these sceptical conclusions, concerning the existence of a First Cause, which an author of great ingenuity has attempted to deduce from it. At present, it is sufficient for my purpose to remark, that the word cause is used, both by philosophers and the vulgar, in two senses, which are widely different.-When it is said, that every change in nature indicates the operation of a cause, the word cause expresses something which is supposed to be necessarily connected with the change; and without which it could not have happened. This may be called the metaphysical meaning of the word; and such causes may be called metaphysical or efficient causes. In natural philosophy, however, when we speak of one thing being the cause of another, all that we mean is, that the two are constantly conjoined; so that when we see the one we may expect the other. These conjunctions we learn from experience alone; and without an acquaintance with them we could not accommodate our conduct to the established course of nature. The causes which are the objects of our investigation in natural philosophy, may, for the sake of distinction, be called physical causes.

I am very ready to acknowledge, that this doctrine, concerning the object of natural philosophy, is not altogether agreeable to popular prejudices. When a man, unaccustomed to metaphysical speculations, is told, for the first time, that the science of physics gives us no information concerning the efficient causes of the phenomena about which it is employed, he feels some degree of surprise and mortification. The natural bias of the mind is surely to conceive physical events as somehow linked together; and material substances, as possessed of certain powers and virtues, which fit them to produce particular effects. That we have no reason to believe this to be the case, has been shewn in a very particular manner by Mr. Hume, and by other writers; and must, indeed, appear evident to

VOL. II.

1 See Note C.

G