Duration and succession, eternity and time, are not identical. Duration is the continuance in existence of finite creatures, a continuance which is measured by the equable motion of planetary orbs, and imperfectly by phenomenal changes in our mental states. Succession is simply an order of phenomena, the recurrence, at regular or irregular intervals, of like changes, or the series of different states in the same existence. _Time is a certain correlation of successive existences._ Eternity is an attribute of the absolute Being--the _timelessness_ of God. He is not subject to the law of change, and therefore not to the law of time, therefore his absolute being can not be measured by successive epochs. Let us now endeavor to dismiss from our thought all this perplexing necromancy of words, and humbly pray, with Themistocles, for "some sweet voluptuous art of forgetting." Let us fix our mental gaze upon the _objects_ of thought which are denoted by the terms _time_ and _space_, and ask what are they? Are they existences or attributes, are they ideal or real, are they entities or relations? Have we any clear and definite notions of which these are the unequivocal signs? The solution of these questions is the essential condition of a true philosophy of time and space. First of all, is it not self-evident that, if time and space are for us the objects of thought, they must be conceived under the categories of _Being_ or _Quality_ or _Relation_? If they can not be thought as real existences, or as attributes of existing things, or as relations among existing things, they can not be thought at all--they are non-entities, and we can not think about nothing. "Thought can only be realized by thinking _something_ ... this something must be thought as existing ... and we can only think a thing as existing, by thinking it as existing in this or that determinate manner of existence; and whenever we cease to think of something as existing--something existing in a determinate manner of existence--we cease to think at all."[100] McCosh asserts that time and space are "neither substances, modes, nor relations."[101] What, then, are they? He answers, "They seem to be entitled to be put in a class by themselves, and resemble substances, modes, relations only in that they are _existences, entities, realities_."[102] But if they are entitled to be put in a class by themselves, what is the name of that class, and by what characteristic marks shall we distinguish it? If they are realities, they must have being, or inhere in something that has being, or be relations of something in being. If they are existences, they must be the objects of _sense perception_, or _rational intuition_, or _immediate judgment_, otherwise they can not be cognized at all, for "the mind can not create objects of its own cognition."