It is necessary to explain a little this convenience; and the explanation will have another advantage, that it will still farther illustrate the manner in which Predication serves the great purpose of marking the Order of ideas in a Train. If the sensations or ideas in a train were to be marked as merely so many independent items, the mode of marking the order of them would be simple; the order of the marks itself might suffice. If this, for example, were the train; smell of a rose, sight of a rat, sound of a trumpet, touch of velvet, prick of a pin, these names placed in order might denote the order of the sensations. In the greater number of instances, however, it is necessary to mark the train as the train of somebody; and for this purpose additional machinery is required. Suppose that the train I have to mark is the train of John, a train of the sensations of John; what are the marks for which I shall have occasion? It is first of all evident that I must have a mark for John, and a mark for each of the sensations. Suppose it is my purpose to represent John as having a sensation by each of his senses, sight, smell, &c., how must I proceed? I have first the word John, for the mark of the person; and I have the word seeing, for the mark of the sensation. But beside the marks, "John," "seeing," I have occasion for a mark to show that I mean the mark "seeing" to be applied to the mark "John," and not to any other. For that purpose I use the word "is." I say "John is seeing," and the first sensation of John's train is now sufficiently {173} denoted. In the same manner I proceed with the rest; John is smelling, John is tasting, John is hearing, John is touching. But I have often occasion to speak not only of John's present sensations, but of his past or his future sensations; not of John as merely now seeing, hearing, &c., but as having been, or as going to be, the subject of these sensations. The _Copula_ may be so contrived as most commodiously to connote the main distinctions of Time: not merely to mark the connection between the two marks which form the subject and the predicate of the proposition, but to mark, along with this, either past, or present, or future, Time. Thus, if I say John is seeing, the copula marks present time along with the peculiar connection between the predicate and the subject; if I say John was seeing, it connotes past time; if I say John will be seeing, it connotes future time.