The explanation must therefore be sought elsewhere, and is one which also applies to the use of the word ‘winter’ for year etc. Primitive man knows only concrete indications of time, and in reckoning prefers to use a concrete and clearly visible point of reference. The complete day of 24 hours is unknown to him and so he _must_ reckon according to the principle of _pars pro toto_, and as a matter of fact it is possible to reckon just as well from a part of the whole as from the whole itself, provided that the part chosen is one that only recurs once every day. The day itself, with its various occupations, offers no such point of reference unless the reckoning is based upon the daily appearance of the sun, which is also actually done in certain cases. However in the daily course of the sun, as we have already seen, two features, its duration and the changing position of the sun, stand out prominently: but it is easier to reckon from points than from lengths, which divert the attention from the number. Now the sleeping-time is necessarily bound up with each day, yet it has no separate parts, or acquires them only later among certain peoples. The time between going to sleep in the evening and waking in the morning appears as an undivided unit, a point. It offers for reckoning a convenient basis in which no mistake or hesitation is possible such as can occur in the various occupations that fall within the period computed. The method of reckoning in nights is merely an outcome of the necessity for a concrete unmistakable time-indication: it is a typical example of the _pars pro toto_ principle and time-reckoning, which, on the psychological grounds just mentioned are especially favoured in the counting.