All the difference in our sensations are produced by the greater or smaller number, and by the more or less exterior position of the nerves, which is the cause that some of our senses, as the eye, ear, and smell, may be affected by the small particles which exhale from particular bodies; others, as tasting and feeling, require actual contact, or more gross emanations, so as to form a solid mass; and it is this feeling which gives us the sensation of solidity, or fluidity, and of the heat of bodies. A fluid differs from a solid, because it has not any particles gross enough to admit us to grasp it on different sides at one time. The particles which compose fluids cannot touch each other but at one point, or so few points, that no part can have any considerable adliesion with another. Solid bodies, reduced even into an impalpable powder, do not absolutely lose their solidity, because the parts, touching each other by many sides, preserve a degree of cohesion; and this is the reason why we can make them up in masses, and squeeze them together. The sense of feeling is spread over the whole body, but employs itself differently in different parts. The sensation which results from feeling is excited by the contact of some foreign body to that of our own. If we apply a foreign body against the breast or shoulder we shall feel it, but without having a single idea of its form, because the breast or shoulder touches but one side only. It is the same with respect to all other parts which cannot bend themselves round or embrace at one time many parts of foreign bodies. Those parts of our body, which, like the hand, are divided into many flexible and moveable parts, and can apply themselves at one time upon different sides of a foreign body, are those only which can give us the ideas of their form and size.