According to the former view the power of prevision is the result of a mere heightening of the faculties by which we can always foresee the future to some slight extent. If we see a blind man walking towards the edge of a cliff it is not difficult to foresee that he will, probably, fall off it and be smashed at the bottom. Such a sight could easily be supposed to give rise to a visualisation of the corpse at the bottom of the cliff, which might pass for a prophetic vision. In such simple matters it is not difficult to imagine that a suitable clairvoyant state, combined with unconscious but accurate reasoning and subsequent visualisation, would enable the percipient to forecast the future. But clearly the accuracy of such a forecast would depend on the perception of _all_ the factors involved, as well as on the precision of the unconscious reasoning. Hence, although we might readily accept this explanation in the case of prevision of events in the immediate future, or in the case of vague presentiments, it becomes increasingly difficult to do so, as the event prevised becomes more remote and the number of factors which may possibly influence the issue are proportionately increased. I need hardly say that these factors of which I speak must include Psychic states and so forth. To use the terminology to which we have by this time become accustomed, we could, theoretically, forecast the distribution of every particle of matter in three space, provided we knew present distribution and velocities; and provided also that no interference could arise from external, _i.e._, four space, sources. But in order to be certain of the latter, we must know all about four space dispositions and so on to the "N"th degree. Absolute prevision could therefore only result from a complete knowledge of all the factors in _every_ space combined with absolutely perfect reasoning powers. Although, as will be seen, certain of the ideas in the above have a place in what I believe to be the true theory of prevision, the explanation as above described does not appear to me to be satisfying. The heightening of faculty required in all but the very simplest cases is too great to be accepted except in the last resort. Now, as regards the other theory, that the future does actually exist _now_ and that only our own limitations prevent us from apprehending it. Consider again the crude and metaphorical representation of change as resulting from the passage across our three space of a congeries of four space solids which supposes that the distribution of matter at any moment is simply a very thin cross section of this congeries. If this were the case it is evident that to anyone who had the power of moving freely in four dimensions it would be possible to move up the mass and see what some cross section was like which had not yet arrived at our space. This is desperately crude but it gives the general idea.