That a superficial view of matter, as it presents itself to our mere organs of sense, should lead us to form a different opinion, is, however, what might readily be supposed, because the analogies, which that superficial view presents, are of a kind that seem to mark a loss of identity whenever the state itself is altered. In experimental philosophy, and in the obvious natural phenomena of the material world, whenever a body changes its state, some addition or separation has previously taken place. Thus, water becomes steam by the addition, and it becomes ice by the loss, of a portion of that matter of heat which is termed by chemists _caloric_; which loss and addition are, of course, inconsistent with the notion of absolute numerical identity of the corpuscles, in the three states of water as a solid, a liquid, and a gaseous vapour. _Perception_, by which the mind is metaphorically said to acquire knowledge, and _forgetfulness_, by which it is metaphorically said to lose knowledge, have, it must be confessed, a very striking analogy to these processes of corpuscular loss and gain; and, since absolute identity _seems_ to be inconsistent with a change of state in the one set of phenomena, with which we are constantly familiar, we find difficulty in persuading ourselves, that it is not inconsistent with a change of state in the other set also. It is a difficulty of the same kind as that which every one must have felt, when he learned, for the first time, the simple physical law, that matter is indifferent as to the states of motion and rest, and that it requires, therefore, as much force to destroy completely the motion of a body, as to give it that motion when at rest. We have not been accustomed to take into account the effects of _friction_, and of atmospherical resistance, in gradually destroying, without the interference of any visible force, the motion of a ball, which we are conscious of effort in rolling from our hand; and we think, therefore, that rest is the natural state of a body, and that it is the very nature of motion to cease spontaneously. “Dediscet animus sero, quod dedicit diu.” It is a very just saying of a French writer, that “it is not easy to persuade men to put their _reason_ in the place of their _eyes_; and that when, for example, after a thousand proofs, they are reasonable enough to do their best to believe, that the planets are so many opaque, solid, habitable orbs, like our earth, they do not believe it in the same manner as they would have done if they had never looked upon them in another light. There still comes back upon their belief something of the first notion which they had, that clings to them with an obstinacy, which it requires a continual effort to shake off.”[49]