In the whole of physics there is no idea which, for several centuries, has raised such difficulties, and which, even to-day, is so much discussed, as that of the æther. It is not that different schools of physicists have proposed various hypotheses, which they refuse to abandon, but, as the authors have already remarked, it is rather the facts themselves, which appear to contradict one another. The idea that there was a material something which filled all space, and which carried messages from the stars to us, was familiar to Aristotle. But we cannot say that Aristotle's hypothesis was founded on scientific lines, for the ideas which were then prevalent concerning the tasks which the æther had to fulfil, and especially about the nature of light, which the æther served to transmit, were very imperfect. The idea of a material something having definite properties, so that it could fulfil certain definite tasks, originates from the time of Newton.