The process in forming those classes was said to be this. The Mind leaves out of its view this, and that, and the other thing, in which individuals differ from one another; and retaining only those in which they all agree, it forms them into a class. But what is {249} this forming of a class? What does it mean? When I form a material aggregate; when I collect a library; when I build a house; when I even raise a heap of stones; I move the things, whatever they may be, and place them, either regularly or irregularly, in a mass together. But when I form a class, I perform no operation of this sort. I touch not, nor do I in any way whatsoever act upon the individuals which I class. The proceeding is all mental. Forming a class of individuals, is a mode of regarding them. But what is meant by a mode of regarding things? This is mysterious; and is as mysteriously explained, when it is said to be the taking into view the particulars in which individuals agree. For what is there, which it is possible for the mind to take into view, in that in which individuals agree? Every colour is an individual colour, every size is an individual size, every shape is an individual shape. But things have no individual colour in common, no individual shape in common, no individual size in common; that is to say, they have neither shape, colour, nor size in common. What, then, is it which they have in common, which the mind can take into view? Those who affirmed that it was something, could by no means tell. They substituted words for things; using vague and mystical phrases, which, when examined, meant nothing. Plato called it [Greek: i)de/a], Aristotle, [Greek: ei)=dos], both, words taken from the verb to see; intimating, something as it were seen, or viewed, as we call it. At bottom, Aristotle's [Greek: ei)=dos], is the same with Plato's [Greek: i)de/a], though Aristotle makes a great affair of some very trifling differences, which he creates and sets up between them. The Latins, translated both [Greek: i)de/a], and {250} [Greek: ei)=dos], by the same words, and were very much at a loss for one to answer the purpose; they used _species_, derived in like manner from a verb to see, but which, having other meanings, was ill adapted for a scientific word; they brought, therefore, another word in aid, _forma_, the same with [Greek: o(/rama], derived equally from a verb signifying to see, which suited the purpose just as imperfectly as _species_; and as writers used both terms, according as the one or the other appeared best to correspond with their meaning, they thickened by this means the confusion.