ed by us as unknown causes or occasions of the sensations, to which we give the same names. But though common sense says nothing of the nature of the qualities, it plainly indicates the existence of them; and to deny that there can be heat and cold when they are not felt, is an absurdity too gross to merit confutation. For what could be more absurd than to say that the thermometer cannot rise or fall unless some person be present, or that the coast of Guinea would be as cold as Nova Zembla if it had no inhabitants.

"It is the business of philosophers to investigate, by proper experiments and induction, what heat and cold are in bodies. And whether they make heat a particular element diffused through nature, and accumulated in the heated body, or whether they make it a certain vibration of the parts of the heated body; whether they determine that heat and cold are contrary qualities, as the sensations undoubtedly are contrary, or that heat only is a quality, and cold its privation; these questions are within the province of philosophy; for common sense says nothing on the one side or the other.

"But, whatever be the nature of that quality in bodies which we call heat, we certainly know this, that it cannot in the least resemble the sensation of heat. It is no less absurd to suppose a likeness between the sensation and the quality, than it would be to suppose that the pain of the gout resembles a square or a triangle. The simplest man that hath common sense does not imagine the sensation of heat, or anything that resembles that sensation, to be in the fire. He only imagines that there is something in the fire which makes him and other sentient beings feel heat. Yet as the name of heat, in common language, more frequently and more properly signifies this unknown something in the fire than the sensation occasioned by it, he justly laughs at the philosopher who denies that there is any heat in the fire, and thinks that he speaks contrary to common sense."

30. Of the sensations of hardness and softness.

"Let us next consider," continues the same writer, HARDNESS and SOFTNESS; by which words we always

understand real properties or qualities of bodies, of which we have a distinct conception.

"When the parts of a body adhere so firmly that it cannot easily be made to change its figure, we call it hard; when its parts are easily displaced, we call it soft. This is the notion which all mankind have of hardness and softness: they are neither sensations nor like any sensation; they were real qualities before they were perceived by touch, and continue to be so when they are not perceived: for if any man will affirm that diamonds were not hard till they were handled, who would reason with him?

"There is, no doubt, a sensation by which we perceive a body to be hard or soft. This sensation of hardness may easily be had by pressing one's hand against a table, and attending to the feeling that ensues, setting aside as much as possible all thought of the table and its qualities, or of any external thing. But it is one thing to have the sensation, and another to attend to it and make it a distinct object of reflection. The first is very easy; the last, in most cases, extremely difficult.

"We are so accustomed to use the sensation as a sign, and to pass immediately to the hardness signified, that, as far as appears, it was never made an object of thought, either by the vulgar or by philosophers; nor has it a name in any language. There is no sensation more distinct or more frequent; yet it is never attended to, but passes through the mind instantaneously, and serves only to introduce that quality in bodies which, by a law of our constitution, it suggests.

"There are, indeed, some cases wherein it is no difficult matter to attend to the sensation occasioned by the hardness of a body; for instance, when it is so violent as to occasion considerable pain: then nature calls upon us to attend to it; and then we acknowledge that it is a mere sensation, and can only be in a sentient being. If a man runs his head with violence against a pillar, I appeal to him whether the pain he feels resembles the hardness of the stone; or if he can conceive anything like what he feels to be in an inanimate piece of matter.

"The attention of the mind is here entirely turned to

wards the painful feeling; and, to speak in the common language of mankind, he feels nothing in the stone, but feels a violent pain in his head. It is quite otherwise when he leans his head gently against the pillar; for then he will tell you that he feels nothing in his head, but feels hardness in the stone. Hath he not a sensation in this case as well as in the other? Undoubtedly he hath; but it is a sensation which nature intended only as a sign of something in the stone; and, accordingly, he instantly fixes his attention upon the thing signified; and cannot, without great difficulty, attend so much to the sensation as to be persuaded that there is any such thing distinct from the hardness it signifies.

"But however difficult it may be to attend to this fugitive sensation, to stop its rapid progress, and to disjoin it from the external quality of hardness, in whose shadow it is apt immediately to hide itself: this is what a philosopher by pains and practice must attain, otherwise it will be impossible for him to reason justly upon this subject, or even to understand what is here advanced. For the last appeal, in subjects of this nature, must be to what a man feels and perceives in his own mind."

§ 31. Of certain indefinite feelings sometimes ascribed to the touch. In connexion with these views on the sensations of touch, it is proper to remark, that certain feelings have been ascribed to that sense, which are probably of a character too indefinite to admit of a positive and undoubted classification. Although they clearly have their place in the general arrangement which has been laid down, with the states of mind which we are now considering; that is to say, are rather of an external and material, than of an internal origin; still they do not so evidently admit of an assignment to a particular sense. Those sensations to

which we now refer, (if it be proper to use that term in application to them,) appear to have their origin in the human system considered as a whole, made up of bones, flesh, muscles, the senses, &c., rather than to be susceptible of being traced to any particular part. Of this description are the feelings expressed by the terms uneasiness, weariness, weakness, sickness, and those of an opposite character, as ease, hilarity, health, vigour, &c.

Similar views will be found to apply, in part at least, to the sensations which we express by the terms HUNGER and THIRST. These appear to be complex in their nature, including a feeling of uneasiness, combined with a desire to relieve that uneasiness. When we say that these views will apply in part to hunger and thirst, the design is to limit the application of them to the element of uneasiness. This elementary feeling undoubtedly has its origin in the bodily system, and therefore comes in this case under the general class of notions of an EXTERNAL origin; but still it is not easy to say that it should be arranged with our tactual feelings, which has sometimes been done. Every one must be conscious, it is thought, that the feeling of hunger does not greatly resemble the sensations of hardness and softness, roughness or smoothness, or other sensations which are usually ascribed to the touch.The cause of that peculiar state of the nerves of the stomach, which is antecedent to the uneasy feeling involved in what is termed hunger, has been a subject of difference of opinion, and does not appear to be well understood. If we were fully acquainted with this we might perhaps be less at a loss where to arrange the feeling in question.

32. Relation between the sensation and what is outwardly signified. We here return a moment to the subject of the relation between the internal sensation and the outward object; and again repeat, that the mental state and the corresponding outward object are altogether diverse. This view holds good in the case of the secondary, as well as of the primary qualities of matter. Whether we speak of extension, or resistance, or heat, or colour, or roughness, there are, in all cases alike, two things, the internal affection and the outward quality; but they are utterly distinct, totally without likeness to each other. But how it happens that one thing which is totally different from another can nevertheless give us a knowledge of that from which it differs, it would be a waste of time to attempt to explain. Our knowledge is undoubtedly limited to the mere fact. This is one those of difficult but decisive points in MENTAL PHILOSOPHY, of which it is essential to possess a

precise and correct understanding. The letters which cover over the page of a book are a very different thing from the thought and the combinations of thought which they stand for. The accountant's columns of numerals are not identical with the quantities and their relations which they represent. And so in regard to the mind; all its acts are of one kind, and what they stand for is of another. The mind, in all its feelings and operations, is governed by its own laws, and characterizes its efforts by the essential elements of its own nature. Nothing which is seen or heard, nothing which is the subject of taste, or touch or any other sense, nothing material, which can be imagined to exist in any place or in any form, can furnish the least positive disclosure either of its intrinsic nature or of the mode of its action.

What, then, is the relation between the sensation and the outward object, between the perception and the thing perceived? Evidently that of the sign and the thing signified. And as, in a multitude of cases, the sign may give a knowledge of its objects without any other grounds of such knowledge than mere institution or appointment, so it is in this. The mind, maintaining its appropriate action, and utterly rejecting the intervention of all images and visible representations, except what are outward and material, and totally distinct from itself both in place and nature, is, notwithstanding, susceptible of the knowledge of things exterior, and can form an acquaintance with the universe of matter.

CHAPTER VI.

THE SENSE OF SIGHT.

33. Of the organ of sight, and the uses or benefits of that sense. Of those instruments of external perception with which a benevolent Providence has favoured us, a high rank must be given to the sense of seeing. If we were restricted in the process of acquiring knowledge to the informations of the touch merely, how many embarrassments