But, as Einstein explained to me, even under such difficult circumstances, it would be possible to build up a science, for the reason that phenomena, as far as they are perceptible, may be transformed so that they become manifest to other senses if one sense should be absent. For example, the electrical conductivity of selenium is strongly influenced by the amount of illumination that falls on it. Thus light acts on a selenium cell, causing changes of current intensity, which in their turn may be perceived by feeling, or by chemical action on the mucous fluid of the tongue. Ultimately we are concerned only with a differentiation that enables us to refer identical experiences to identical events. We should certainly encounter enormous difficulties in endeavouring to form a physical picture of our surrounding world if the number of our senses should become less than the organs with which we actually operate. Yet, in principle, we should be able to overcome all difficulties by means of much lengthened and complicated lines of research, even if we should have only a single sense left, or if we had only one at the very outset. The construction of a Science would then be possible, and would give the same results, although it might be propounded only after a delay of perhaps millions of years. [It is naturally assumed that the intellect is retained, as this is the necessary condition for all scientific research. Since the degree of understanding depends on the senses--_nihil est in intellectu, quod non prius fuerit in sensu_--we may conjecture that a human being with only one sense organ would work with a minimum degree of understanding, which would be insufficient for the acquirement of any knowledge whatsoever. This transcendental question, which lies almost beyond the bounds of discussion, was not touched on in our conversation, as the subject was restricted so that it should not drift into metaphysical regions. Nevertheless, I should like to mention that a speculation of this kind is recorded in the history of science. Condillac, in a study teeming with ideas, investigates the behaviour of a "Statue," that he represents as a human being, with the assumption that there is at first no idea in the soul of this statue-person. This living creature is enclosed in a marble envelope, the sole exterior organ of which is at first the organ of smell. He then shows that by means of this single sense all manner of sensations and expressions of will may develop in his "statue." Condillac does not, however, undertake to give a convincing proof that this creature, restricted to the organ of smell, would be able to discover physically the relationships that hold in physical nature, and thus to build up a scientific system. Thus Einstein, in his discussion, goes considerably further than the author of this statue.] * * * * * * * * Has the "eternal repetition," as outlined by Nietzsche, any meaning?