If mankind are never to think of that which is behind appearance--can never think of a reality that is behind what they see--because their forms of thought are molded on experiences of phenomena that they see, and because the connotations of their words express the relations of those phenomena and no others, a vast domain of thinking is necessarily closed to them. This is not the experience of our minds. Every day of our lives we go on in search of that which is beyond appearance, and we find it. Take again, for example, the phenomena of a measurable portion of space or time. What appears to us gives an idea of space and time. We measure as great a portion of either as our forms of expression admit of our describing by definite terms, but we are immediately conscious of another reality, an endless extension or duration, because we are conscious that we have not exhausted and can not exhaust, by our measurements and descriptions, the whole possible existence of space or time. This new reality behind appearance is just as truly thinkable, just as true a consciousness, as is the measurable portion of time or space; for it is time or space of which we are constantly thinking, whether it is an extent or duration which we can describe in words, or whether we can only say that it is extent or duration without beginning and without end. Our minds are so constituted that the existence which is manifested to us by observable phenomena leads us to go behind the appearance in search of another reality beyond that which is manifested by the phenomena that we see. All that is inscrutable about this other reality that lies behind appearance is that we can not understand how it came to be, any more than we can understand how the phenomenon which we see and can measure and describe in a definite form came to exist. We do not bring, and do not need to bring, this other reality into connection with appearance. We first have an idea of space and time from observable and measurable phenomena. The reality of extension without limit, and duration without end, follows of necessity, by a process of thought which we can not escape. But now it becomes needful to answer a further objection. I have said that we are all the while thinking of space, whether it is a measurable and limited or an immeasurable and illimitable space. Mr. Spencer, anticipating this obvious statement, admits that the form of relation between the two ideas, although "almost blank," preserves a certain qualitative character; that is, it is of the quality of space of which we think, whether it is measurable or immeasurable, and therefore it remains "a vaguely identifiable relation." But when, in place of one of the terms of the relation qualitatively the same as the other, we substitute an existence that can not be defined, and is therefore both quantitatively and qualitatively unrepresentable, the relation, he asserts, lapses entirely; one of the terms becomes wholly "unknowable."