First, let us enquire what we mean by saying that fire is hot ; and about this we may reason from the dividing or cutting power which it exercises on our bodies. We all of us feel that fire is sharp ; and we may further consider the fineness of the sides, and the sharpness of the angles, and the smallness of the particles, and the swiftness of the motion — all this makes the action of fire violent and sharp, so that it cuts whatever it meets. And we must not forget that the original figure of fire (i.e. the pyramid), more than any other form, has a dividing power which cuts our bodies into small pieces (Kepmatizei), and thus naturally produces that affection which we call heat ; and hence the origin of the name (thepmos, Kepma). Now, the opposite of this is sufficiently manifest ; nevertheless we will not fail to describe it. For the larger particles of moisture which surround the body, entering in and driving out the lesser, but not being able to take their places, compress the moist principle in us ; and this from being unequal and disturbed, is forced by them into a state of rest, which is due to equability and compression. But things which are contracted contrary to nature are by nature at war, and force themselves apart ; and to this war and convulsion the name of shivering and trembling is given ; and the whole affection and the cause of the affection are both termed cold. That is called hard to which our flesh yields, and soft which yields to our flesh ; and things are also termed hard and soft relatively to one another. That which yields has a small base ; but that which rests on quadrangular bases is firmly posed and belongs to the class which offers the greatest resistance ; so too does that which is the most compact and therefore most repellent. The nature of the light and the heavy will be best understood when examined in connexion with our notions of above and below ; for it is quite a mistake to suppose that the universe is parted into two regions, separate from and opposite to each other, the one a lower to which all things tend which have any bulk, and an upper to which things only ascend against their will. For as the universe is in the form of a sphere, all the extremities, being equidistant from the centre, are equally extremities, and the centre, which is equidistant from them, is equally to be regarded as the opposite of them all. Such being the nature of the world, when a person says that any of these points is above or below, may he not be justly charged with using an improper expression ? For the centre of the world cannot be rightly called either above or below, but is the centre and nothing else ; and the circumference is not the centre, and has in no one part of itself a different relation to the centre from what it has in any of the opposite parts. Indeed, when it is in every direction similar, how can one rightly give to it names which imply opposition ? For if there were any solid body in equipoise at the centre of the universe, there would be nothing to draw it to this extreme rather than to that, for they are all perfectly similar ; and if a person were to go round the world in a circle, he would often, when standing at the antipodes of his former position, speak of the same point as above and below ; for, as I was saying just now, to speak of the whole which is in the form of a globe as having one part above and another below is not like a sensible man.
The reason why these names are used, and the circumstances under which they are ordinarily applied by us to the division of the heavens, may be elucidated by the following supposition : — if a person were to stand in that part of the universe which is the appointed place of fire, and where there is the great mass of fire to which fiery bodies gather — if, I say, he were to ascend thither, and, having the power to do this, were to abstract particles of fire and put them in scales and weigh them, and then, raising the balance, were to draw the fire by force towards the uncongenial element of the air, it would be very evident that he could compel the smaller mass more readily than the larger ; for when two things are simultaneously raised by one and the same power, the smaller body must necessarily yield to the superior power with less reluctance than the larger ; and the larger body is called heavy and said to tend downwards, and the smaller body is called light and said to tend upwards. And we may detect ourselves who are upon the earth doing precisely the same thing. For we of separate earthy natures, and sometimes earth itself, and draw them into the uncongenial element of air by force and contrary to nature, both clinging to their kindred elements. But that which is smaller yields to the impulse given by us towards the dissimilar element more easily than the larger ; and so we call the former light, and the place towards which it is impelled we call above, and the contrary state and place we call heavy and below respectively. Now the relations of these must necessarily vary, because the principal masses of the different elements hold opposite positions ; for that which is light, heavy, below or above in one place will be found to be and become contrary and transverse and every way diverse in relation to that which is light, heavy, below or above in an opposite place. And about all of them this has to be considered : — that the tendency of each towards its kindred element makes the body which is moved heavy, and the place towards which the motion tends below, but things which have an opposite tendency we call by an opposite name. Such are the causes which we assign to these phenomena. As to the smooth and the rough, any one who sees them can explain the reason of them to another. For roughness is hardness mingled with irregularity, and smoothness is produced by the joint effect of uniformity and density.
The most important of the affections which concern the whole body remains to be considered — that is, the cause of pleasure and pain in the perceptions of which I have been speaking, and in all other things which are perceived by sense through the parts of the body, and have both pains and pleasures attendant on them. Let us imagine the causes of every affection, whether of sense or not, to be of the following nature, remembering that we have already distinguished between the nature which is easy and which is hard to move ; for this is the direction in which we must hunt the prey which we mean to take. A body which is of a nature to be easily moved, on receiving an impression however slight, spreads abroad the motion in a circle, the parts communicating with each other, until at last, reaching the principle of mind, they announce the quality of the agent. But a body of the opposite kind, being immobile, and not extending to the surrounding region, merely receives the impression, and does not stir any of the neighbouring parts ; and since the parts do not distribute the original impression to other parts, it has no effect of motion on the whole animal, and therefore produces no effect on the patient. This is true of the bones and hair and other more earthy parts of the human body ; whereas what was said above relates mainly to sight and hearing, because they have in them the greatest amount of fire and air. Now we must conceive of pleasure and pain in this way. An impression produced in us contrary to nature and violent, if sudden, is painful ; and, again, the sudden return to nature is pleasant ; but a gentle and gradual return is imperceptible and vice versa. On the other hand the impression of sense which is most easily produced is most readily felt, but is not accompanied by Pleasure or pain ; such, for example, are the affections of the sight, which, as we said above, is a body naturally uniting with our body in the day-time ; for cuttings and burnings and other affections which happen to the sight do not give pain, nor is there pleasure when the sight returns to its natural state ; but the sensations are dearest and strongest according to the manner in which the eye is affected by the object, and itself strikes and touches it ; there is no violence either in the contraction or dilation of the eye. But bodies formed of larger particles yield to the agent only with a struggle ; and then they impart their motions to the whole and cause pleasure and pain — pain when alienated from their natural conditions, and pleasure when restored to them. Things which experience gradual withdrawings and emptyings of their nature, and great and sudden replenishments, fail to perceive the emptying, but are sensible of the replenishment ; and so they occasion no pain, but the greatest pleasure, to the mortal part of the soul, as is manifest in the case of perfumes. But things which are changed all of a sudden, and only gradually and with difficulty return to their own nature, have effects in every way opposite to the former, as is evident in the case of burnings and cuttings of the body.