It is from the union of these two principles that resulted genethlialogy, or the science by which the point of departure being known in any sphere whatever, they believed they had discovered, by the aspect and direction of the stars, the portion of this sphere which must immediately follow this point. But this union, outside of the enormous difficulty that it presented, still involved in its execution very dangerous consequences. This is why they concealed in the sanctuaries the science which was its object, and made of religion a secret and state affair. The prevision of the future, supposing it possible as the ancients did, is not, in effect, a science that one should abandon to the vulgar, who, being unable to acquire previously the learning necessary, and having but rarely the wisdom which regulates its use, risked debasing it, or making use of it wrongfully. Furthermore, the pontiffs, who were in sole charge, initiated in the great mysteries and possessing the _ensemble_ of the doctrine, knew very well that the future, such even as they could hope to understand it in the perfection of the science, was never aught but a doubtful future, a sort of canvas upon which the power of the Will might exercise itself freely, in such a manner that, although the matter might be determined beforehand, its form was not, and that such an imminent event could be suspended, evaded, or changed by a coöperation of the acts of the will, inaccessible to all prevision. This is what was said with such profoundness by Tiresias, the most famous hierophant of Greece and whom Homer called the only sage,[450] these words so often quoted and so little understood: “Whatever I may see will come to pass, or it will not come to pass”[451]; that is to say, The event that I see is in the necessity of Destiny and it will come to pass, unless it is changed by the power of the Will; in which case it will not come to pass. 17. _That which thou dost not know, pretend not that thou dost. Instruct thyself: for time and patience favour all._ Lysis has enclosed in these two lines the summary of the doctrine of Pythagoras regarding science: according to this philosopher, all science consists of knowing how to distinguish what one does not know and of desiring to learn that of which one knows nothing.[452] Socrates had adopted this idea, as simple as profound; and Plato has consecrated several of his dialogues to its development.[453]