The doctrines which prevented the real meaning of Essences from being understood, not having assumed so settled a shape in the time of Aristotle and his immediate followers as was afterwards given to them by the Realists of the middle ages, we find a nearer approach to a rational view of the subject in the writings of the ancient Aristotelians than in their more modern followers. Porphyry, in his _Isagoge_, approached so near to the true conception of essences, that only one step remained to be taken, but this step, so easy in appearance, was reserved for the Nominalists of modern times. By altering any property, not of the essence of the thing, you merely, according to Porphyry, made a difference in it; you made it ἀλλοῖον: but by altering any property which was of its essence, you made it _another thing_, ἄλλο.(21) To a modern it is obvious that between the change which only makes a thing different, and the change which makes it _another thing_, the only distinction is that in the one case, though changed, it is still called by the same name. Thus, pound ice in a mortar, and being still called ice, it is only made ἀλλοῖον: melt it, and it becomes ἄλλο, another thing, namely, water. Now it is really the same thing, _i.e._ the same particles of matter, in both cases; and you cannot so change anything that it shall cease to be the same thing in this sense. The identity which it can be deprived of is merely that of the name: when the thing ceases to be called ice, it becomes _another thing_; its essence, what constituted it ice, is gone; while, as long as it continues to be so called, nothing is gone except some of its accidents. But these reflections, so easy to us, would have been difficult to persons who thought, as most of the Aristotelians did, that objects were made what they were called, that ice (for instance) was made ice, not by the possession of certain properties to which mankind have chosen to attach that name, but by participation in the nature of a certain _general substance_, called _Ice in general_, which substance, together with all the properties that belonged to it, _inhered_ in every individual piece of ice. As they did not consider these universal substances to be attached to all general names, but only to some, they thought that an object borrowed only a part of its properties from an universal substance, and that the rest belonged to it individually: the former they called its essence, and the latter its accidents. The scholastic doctrine of essences long survived the theory on which it rested, that of the existence of real entities corresponding to general terms; and it was reserved for Locke, at the end of the seventeenth century, to convince philosophers that the supposed essences of classes were merely the signification of their names; nor, among the signal services which his writings rendered to philosophy, was there one more needful or more valuable.(22)