His introduction to Euclid took place accidentally in 1629 (Aubrey's _Lives_, p. 604). Euclid's manner of proof became the model for his own way of thinking upon all subjects. It is less easy to determine when he awoke to an interest in the physical doctrine of motion. The story told by himself (_Vit._ p. xx.) is that, hearing the question asked "What is sense?" he fell to thinking often on the subject, till it suddenly occurred to him that if bodies and their internal parts were at rest, or were always in the same state of motion, there could be no distinction of anything, and consequently no sense; the cause of all things must therefore be sought in diversity of movements. Starting from this principle he was driven to geometry for insight into the ground and modes of motion. The biographies we possess do not tell us where or when this great change of interest occurred. Nothing is said, however, which contradicts a statement that on his third journey in Europe he began to study the doctrine of motion more seriously, being interested in it before; and as he claims more than once (_L.W._ v. 303; _E.W._ vii. 468) to have explained light and sound by a mechanical hypothesis as far back as 1630, the inspiration may be assigned to the time of the second journey. But it was not till the third journey that the new interest became an overpowering passion, and the "philosopher" was on his way home before he had advanced so far as to conceive the scheme of a system of thought to the elaboration of which his life should henceforth be devoted.