No matter how definite and concrete the habitual imagery of a given mind may be, the things represented appear always surrounded by their fringe of relations, and this is as integral a part of the mind's object as the things themselves are. We come, by steps with which everyone is sufficiently familiar, to think of whole classes of things as well as of single specimens; and to think of the special qualities or attributes of things as well as of the complete things--in other words, we come to have _universals_ and _abstracts_, as the logicians call them, for our objects. We also come to think of objects which are only _problematic_, or not yet definitely representable, as well as of objects imagined in all their details. An object which is problematic is defined by its relations only. We think of a thing _about_ which certain facts must obtain. But we do not yet know how the thing will look when realized--that is, although conceiving it we cannot _imagine_ it. We have in the relations, however, enough to individualize our topic and distinguish it from all the other meanings of our mind. Thus, for example, we may conceive of a perpetual-motion machine. Such a machine is a _quæsitum_ of a perfectly definite kind,--we can always tell whether the actual machines offered us do or do not agree with what we mean by it. The natural possibility or impossibility of the thing never touches the question of its conceivability in this problematic way. 'Round-square,' again, or 'black-white-thing,' are absolutely definite conceptions; it is a mere accident, as far as conception goes, that they happen to stand for things which nature never shows us, and of which we consequently can make no picture.