Such is, in the main, the question which you put when you bring in a philosopher Paul, who lives before Peter and has to picture to himself the conditions under which Peter will act. You thus materialize these conditions; you make the time to come into a road already marked out across the plain, which we can contemplate from the top of the mountain, even if we have not traversed it and are never to do so. But, now, you soon notice that the knowledge of the part M O of the curve would not be enough, unless you were shown the position of the points of this line, not only in relation to one another, but also in relation to the points of the whole line M O X Y; which would amount to being given in advance the very elements which have to be determined. So you then alter your hypothesis; you realize that time does not require to be seen, but to be lived; and hence you conclude that, if your knowledge of the line M O was not a sufficient datum, the reason must have been that you looked at it from the outside instead of identifying yourself with the point M, which describes not only M O but also the whole curve, and thus making its movement your own. Therefore, you persuade Paul to come and coincide with Peter; and naturally, then, it is the line M O X Y which Paul traces out in space, since, by hypothesis, Peter describes this line. But in no wise do you prove thus that Paul foresaw Peter's action; you only show that Peter acted in the way he did, since Paul became Peter. It is true that you then come back, unwittingly, to your former hypothesis, because you continually confuse the line M O X Y in its tracing with the line M O X Y already traced, that is to say, time with space. After causing Paul to come down and identify himself with Peter as long as was required, you let him go up again and resume his former post of observation. No wonder if he then perceives the line M O X Y complete: he himself has just been completing it. [Sidenote: Confusion arising from prediction of astronomical phenomena.] What makes the confusion a natural and almost an unavoidable one is that science seems to point to many cases where we do anticipate the future. Do we not determine beforehand the conjunctions of heavenly bodies, solar and lunar eclipses, in short the greater number of astronomical phenomena? Does not, then, the human intellect embrace in the present moment immense intervals of duration still to come? No doubt it does; but an anticipation of this kind has not the slightest resemblance to the anticipation of a voluntary act. Indeed, as we shall see, the reasons which render it possible to foretell an astronomical phenomenon are the very ones which prevent us from determining in advance an act which springs from our free activity. For the future of the material universe, although contemporaneous with the future of a conscious being, has no analogy to it. [Sidenote: Illustration from hypothetical acceleration of physical movements.]