M.: It gives me enough satisfaction, Professor, that you conceded me before a certain limited chance of finding a last refuge in occultism. And even if you, yourself, as a representative of the most rigorous research of physical reality, refuse to consider it, yet the fact that many others are drawn irresistibly towards mysterious phenomena cannot be denied. Should one feel shame on this account? I believe that, in this matter, we are touching on inner confessions that are quite independent of the standard of the mind in which they are embedded. Newton considered the key of the universe to be a personal God, whereas Laplace proclaimed: _Dieu--je n'avais pas besoin de cette hypothèse_: this contrast allows no inference to be drawn as to their relative keenness of mind. And probably the same may be said of the question whether there are other hidden universes besides the one in which we live. In any case, those who feel enthusiasm for such questions can quote in their support good names from the learned world. Immanuel Kant occupied himself seriously and intensively with the wonders of Swedenborg, Kepler practised Astrology, in which he had a firm belief, Roger Bacon, Cardanus, Agrippa, Nostradamus, van Helmont, Pascal, and, among the modern, Fechner, Wallace, Crookes, are to be counted among the mystics. No matter whether the views they held were theosophical, occult, four-dimensional in the spiritistic sense, or coloured by any other superstition; they proclaimed that things that could be rigorously proved were, alone, insufficient for them. Out of presentiment and conjecture they constructed wings with which to fly into regions _extra naturam_. This is how it happened that, as the common folk could not find a place in science for many extraordinary achievements, they assigned their authors to the realm of magicians, as in the case of Paracelsus, Albertus Magnus, Raimundus Lullus, Sylvester II, who were regarded as sorcerers. And this coin is still current: to Edison, of our times, the term, "sorcerer of Menlo-Park," has become attached. In the minds of the populace discovery and invention, works of genius and supernatural phenomena, become confused and indistinguishable; it may even happen to you. Professor, that your works will become invested with legend. I should not like to conjure up what your fate would have been if your theory of relativity had originated at the time of the Inquisition. For the views put forward by Giordano Bruno are mere child's play compared with your theory of the universe as a quasi-spherical closed space of hyper-Euclidean character. The tribunal of the Inquisition would not have understood your differential equation, gravitational potentials, tensors, and equivalence theory; they would abruptly have declared the whole theory to be a magical formula or a manifestation of the devil, and would have honoured it and you with a funeral pyre.