The attribute of =duration= has not yet been defined. It is the bare going on, going forward, keeping like itself, that may be observed in any and every sensation; you recognise it most easily, perhaps, if you listen to a tone, or attend to the kinæsthetic complex as you slowly extend your arm from the elbow. It is _the elementary time-factor in all our perceptions of time_,—in the perceptions of period, of interval, of rate, of rhythm, and so on; though in some of these perceptions it is overlaid and obscured by other factors. Qualitative perceptions undergo relatively little change, just because they are qualitative perceptions; the best and easiest way to _mean_ a quality is to _be_ it; the best way to mean the coffee-taste is to be the coffee-taste; and so our perception of that taste remains practically the same all our life long. Time-perceptions, on the other hand,—and the same thing is true of space-perceptions,—change enormously; the nervous system finds all manner of short-cuts to the meaning of time; and these short-cuts have to be _un_practised, to be practised out, if we are to observe the perception in its original form. Thus, to take the simplest case, a period of time may seem long because the kinæsthetic strain of waiting becomes intense, or because a great number of perceptions and ideas occur during its course; the strain and the number of ideas have come to _mean_ length of time, and the primary experience of duration, so to say, drops out of sight. If, therefore, we wish our observers in the laboratory to compare periods of time; if we wish to find out accurately what durations can be grasped by the attention and held in the memory; then we must break them of these time-habits, and must somehow train them to disregard strain and to discard imagery. We cannot often carry the unravelling of a perception to the very end, though we can go some distance behind the appearances of everyday life. The attribute of =extension= is the bare character of patch or spread that inheres in all sensations from eye and skin, and possibly also in kinæsthetic and organic sensations. No point of light or pressure is so fine that it is not areal. Extension is _the elementary space-factor in all our perceptions of space_. It enters most obviously into the perception of surface, as duration enters most obviously into that of period; but it is the basis also of our perceptions of form, size, distance, locality, direction. Like duration, it is often obscured and overlaid by other factors.