which it usually occupies. The condenfing engine, and what is founded upon it, the wind-gun, fufficiently demonftrate this: and even without the help of fuch ingenious and expenfive machines, we may cafily fatisfy ourselves of the truth of it, by fqueezing a fullblown bladder of which the neck is well tied.

The hardness or softness of bodies, or the greater or smaller force with which they refift any change of shape, feems to depend altogether upon the ftronger or weaker degree of cohesion with which their parts are mutually attracted to one another. The greater or smaller force with which they refift compreffion may, upon many occafions, be owing partly to the fame caufe: but it may likewife be owing to the greater or fmaller proportion of empty space comprehended within their dimenfions, or intermixed with the folid parts which compose them. A body which comprehended no empty space within its dimenfions, which, through all its parts, was completely filled with the refifting fubftance, we are naturally disposed to conceive as fomething which would be abfolutely incompreffible, and which would refift, with unconquerable force, every attempt to reduce it within narrower dimenfions. If the folid and resisting fubftance, without moving out of its place, should admit into the fame place another folid and refifting fubftance, it would from that moment, in our apprehenfion, ceafe to be a folid and refifting fubstance, and would no longer appear to poffefs that quality, by which alone it is made known to us, and which we therefore confider as conftituting its nature and effence, and as altogether infeparable from Hence our notion of what has been called impenetrability of matter; or of the abfolute impoffibility that two folid refifting fubftances fhould occupy the fame place at the fame time.

it.

This doctrine, which is as old as Leucippus, Democritus, and Epicurus, was in the last century revived by Gaffendi, and has fince

been adopted by Newton and the far greater part of his followers, It may at present be confidered as the established system, or as the fyftem that is moft in fashion, and moft approved of by the greater part of the philofophers of Europe. Though it has been opposed by feveral puzzling arguments, drawn from that species of metaphysics which confounds every thing and explains nothing, it seems upon the whole to be the moft fimple, the most diftinct, and the most comprehenfible account that has yet been given of the phænomena which are meant to be explained by it. I fhall only observe, that whatever fyftem may be adopted concerning the hardness or softnefs, the fluidity or folidity, the compreffibility or incompreffibility, of the resisting substance, the certainty of our distinct sense and feeling of its Externality, or of its entire independency upon the organ which perceives it, or by which we perceive it, cannot in the smallest degree be affected by any fuch fyftem. I fhall not therefore attempt to give any further account of such systems.

Heat and cold being felt by almost every part of the human body, have commonly been ranked along with folidity and refistance, among the qualities which are the objects of Touch. It is not, however, I think, in our language proper to say that we touch, but that we feel, the qualities of heat and cold. The word feeling, though in many cafes we use it as fynonimous to touching, has, however, a much more extenfive fignification, and is frequently employed to denote our internal, as well as our external, affections. We feel hunger and thirst, we feel joy and forrow, we feel love and hatred.

Heat and cold, in reality, though they may frequently be perceived by the fame parts of the human body, conftitute an order of fenfations altogether different from those which are the proper objects of Touch. They are naturally felt, not as preffing upon the

organ,

organ, but as in the organ. What we feel while we ftand in the funshine during a hot, or in the fhade during a frofty, day, is evidently felt, not as preffing upon the body, but as in the body. It does not neceffarily fuggeft the presence of any external object, nor could we from thence alone infer the existence of any fuch object. It is a sensation which neither does nor can exist any where but either in the organ which feels it, or in the unknown principle of perception, whatever that may be, which feels in that organ, or by means of that organ. When we lay our hand upon a table, which is either heated or cooled a good deal beyond the actual temperature of our hand, we have two diftinct perceptions: first, that of the solid or resisting table, which is neceffarily felt as something external to, and independent of, the hand which feels it: and fecondly, that of the heat or cold, which by the contact of the table is excited in our hand, and which is naturally felt as nowhere but in our hand, or in the principle of perception which feels in our hand.

But though the fenfations of heat and cold do not neceffarily fuggeft the presence of any external object, we foon learn from experience that they are commonly excited by fome fuch object; sometimes by the temperature of fome external body immediately in contact with our own body, and fometimes by fome body at either a moderate or a great distance from us; as by the fire in a chamber, or by the fun in a Summer's day. By the frequency and uniformity of this experience, by the custom and habit of thought which that frequency and uniformity neceffarily occafion, the Internal Senfation, and the External Cause of that Sensation, come in our conception to be so strictly connected, that in our ordinary and careless way of thinking, we are apt to confider them as almost one and the fame thing, and therefore denote them by one and the fame word. The confufion, however, is in this cafe more in the word than in

the thought; for in reality we ftill retain fome notion of the dif tinction, though we do not always evolve it with that accuracy which a very flight degree of attention might enable us to do. When we move our hand, for example, along the furface of a very hot or of a very cold table, though we fay that the table is hot or cold in every part of it, we never mean that, in any part of it, it feels the fenfations either of heat or of cold, but that in every part of it, it poffeffes the power of exciting one or other of those fenfations in our bodies. The philofophers who have taken so much pains to prove that there is no heat in the fire, meaning that the fenfation or feeling of heat is not in the fire, have laboured to refute an opinion which the most ignorant of mankind never entertained. But the fame word being, in common language, employed to fignify both the sensation and the power of exciting that fenfation, they, without knowing it perhaps, or intending it, have taken advantage of this ambiguity, and have triumphed in their own fuperiority, when by irresistible arguments they establish an opinion which, in words indeed, is diametrically oppofite to the most obvious judgments of mankind, but which in reality is perfectly agreeable to thofe judgments.

Of the Senfe of TASTING.

WHEN We taste any folid or liquid fubftance, we have always two diftinct perceptions; first, that of the folid or liquid body, which is naturally felt as preffing upon, and therefore as external to, and independent of, the organ which feels it; and fecondly, that of the particular taste, relish, or favour which it excites in the palate or organ of Tafting, and which is naturally felt, not as preffing upon, as external to, or as independent of, that organ; but as altogether

in the organ, and nowhere but in the organ, or in the principle of perception which feels in that organ. When we fay that the food which we eat has an agreeable or difagreeable tafte in every part it, we do not thereby mean that it has the feeling or sensation of taste in any part of it, but that in every part of it, it has the power of exciting that feeling or sensation in our palates. Though in this cafe we denote by the fame word (in the fame manner, and for the fame reason, as in the cafe of heat and cold) both the fenfation and the power of exciting that fenfation, this ambiguity of language misleads the natural judgments of mankind in the one cafe as little as in the other. Nobody ever fancies that our food feels its own agreeable or difagreeable tafte.

Of the Senfe of SMELLING.

EVERY smell or odour is naturally felt as in the noftrils; not as preffing upon or refifting the organ, not as in any respect external to, or independent of, the organ, but as altogether in the organ, and nowhere else but in the organ, or in the principle of perception which feels in that organ. We foon learn from experience, however, that this fenfation is commonly excited by fome external body; by a flower, for example, of which the abfence removes, and the prefence brings back, the fenfation. This external body we confider as the cause of this fenfation, and we denominate by the fame words both the sensation and the power by which the external body produces this fenfation. But when we fay that the fmell is in the flower, we do not thereby mean that the flower itself has any feeling of the sensation which we feel; but that it has the power of exciting this fenfation in our noftrils, or in the principle of perception which feels in our noftrils. Though the sensation, and the power by which it is

excited,