_What_ the nature of the change is, that is produced at the extremity of the nerve, it is beyond our power to state, or even to guess; and we are equally ignorant of the manner in which this affection of the nerve is communicated, or is supposed to be communicated, to the brain. But that some affection is gradually propagated, from the one to the other, so as to render the change in the state of the brain subsequent, by a certain interval, to the change in the state of the nerve, is universally believed. In applying to this change the term _impression_, a term indeed which had been in common use before, Dr Reid is careful to point out the reason for which this term appears to him preferable to others; and though I confess that the word seems to me to convey too much the notion of a peculiar well known species of action; that which consists in producing a certain configuration of the object impressed, corresponding with the figure of the impressing object, the very notion that has had so pernicious an effect in the theory of perception; and though I conceive the simple term _change_ or _affection_ to be all which is safely admissible, as long as the nature of the particular change is absolutely unknown; still it must be confessed that _impression_ is a term a little more general than the other names of action, to which Dr Reid alludes, and therefore preferable to them, in the present case. “There is sufficient reason,” he says, “to conclude, that, in perception, the object produces some change in the organ; that the organ produces some change upon the nerve; and that the nerve produces some change in the brain. And we give the name of an _impression_ to those changes, because we have not a name more proper to express, in a general manner, any change produced in a body, by an external cause, without specifying the nature of that change. Whether it be pressure, or attraction, or repulsion, or vibration, or something unknown, for which we have no name, still it may be called an impression. But with regard to the particular kind of this change or impression, philosophers have never been able to discover any thing at all.”[70]