Very much the same is the case with time. Let us admit that the temporal consciousness of our ordinary life is largely mediated by accessory sensations and images. Minutes, hours, days, weeks, are not experienced directly as properties of sense perception, but are extensions of simpler experiences. But such extensions would be impossible if duration and succession were not, somewhere in our mental life, direct experiences. They are direct experiences in some very brief temporal perceptions occupying, say, only a fraction of a second. The flash of a lighthouse signal, the quick succession of sounds when a person knocks at a door, are perceived as having temporal attributes without any mediation by conscious states acting as agents. The _temporal_ attributes are elements of perception no less direct than the intensity of the light or of the sound. The same holds for all other sensations. Time is an attribute common to all. But here, as in space, we cannot tell exactly in what respect all senses are similar so far as the nervous processes are concerned. It seems that these processes or their after effects continue a certain time after the stimulation has ceased. Another attribute common to all sense impressions is the belonging-together of sensations, the _unity in variety_, so to speak. The most striking example is the relationship of tones in harmony and melody. Tones of certain comparatively simple ratios of vibration belong together in a higher degree than others. We cannot explain this by reference to conscious agents mediating the effect. It is a fundamental attribute of each tonal combination, the conscious effect of our inherited nature. It is a property of sense, not of thought. In other cases our consciousness of relationship is indirect, mediated by other conscious agents; for instance, when I group together voluntarily four or five adjoining holes of a sieve and perceive them as a unit. This grouping together would be impossible if the mind did not possess the native ability to perceive a number of sensational elements as a unit without altogether losing the consciousness of variety. It is a mere consequence of our inborn nature when we perceive as such units, for example, an animal romping among unchanging surroundings, a picket fence divided into groups by the fence posts, a familiar compound perfume, a dish made up of several familiar food substances. The same holds for successive elements. We could never perceive tones or noises in various rhythm forms if our mind did not possess the native ability to perceive a number of successive elements of sensation under certain conditions as a sensory unit. Our numerical concepts are obviously only abstract symbols for units containing each a certain variety of elements. 4. _Sensation and Stimulus_