For the purpose of the inquiry we must begin by taking the term thing in the same wide and ambiguous sense in which everyday thought and discourse use it. We must reckon among the things which are the topic of discussion, human persons, animals, plants, greater and smaller inorganic masses, in a word, whatever the most matter-of-fact common-sense thinking recognises as possessed of a character in virtue of which it can as a whole determine the course of experience at a given moment. The character or aspect in virtue of which such a whole determines the course of experience in this one special way rather than another, is by this definition excluded from our conception of a thing; it is not the thing itself but its quality or property or relation to some other thing, and forms the subject of our second and third problems. Thus we may say of a thing, in the sense in which we are using the term, that it is what has existence as a whole here and now in the series of experiences, though in saying so we must be careful to bear in mind that the here and now of the thing’s existence are not indivisible points of space or time, but continuous stretches of extension and duration. Now, when we ask in what sense such a thing is one, and why we mark off the limits which separate the one thing from the other things just where we do, it at once becomes apparent that the oneness is a matter of degree. We seem at first sight able with comparatively little difficulty to decide that the organism of a human being or of one of the higher animals is one thing; when we come to deal with the lower organisms which consist of loosely aggregated colonies of largely independently functioning cells, we begin to feel more diffidence in pronouncing what is one organism, though we still think we can say what is one cell. So, in dealing with inanimate masses, while we might be ready to say without much misgiving that a machine of our own construction is one, we should find it much harder to decide whether what we perceive as a mere inorganic mass is one or many, and harder still to give reasons for our decision in a particular case. And even in the cases where our decision is most unhesitatingly pronounced, subsequent reflection will show that the matter may not be so obvious as it seems. For instance, a pair of separated Siamese twins would undoubtedly be generally held to be two organisms and not one; but whether they were one or two before the severance is a question we should find it easier to ask than to answer.