In his immensely popular L’Irréligion de L'Avenir (The Non-Religion of the Future) J.M. Guyau showed that the soul has been invoked to account (among other things) for the difference between the living and the dead and for dreams. The breath, he said, was the most tangible thing, other than the blood, with which life could be definitely associated. It could even be heard leaving the body at the last gasp. It seems to be immaterial and invisible, so that it is not surprising that in many languages words for soul or spirit are derived from words meaning the breath or the wind. As for dreams, Guyau pointed out that the dreamer experiences adventures, yet is assured by his companions that he did not leave the room or the tent at all during the night, and that this could be explained by supposing that there is a soul that can leave the body, roam the country, and then return to its normal corporeal abode. In dreams one not only goes on adventures, but also meets friends and talks with them, yet thereafter they know nothing of the meeting. Some semblance of them had therefore been separated from their bodies without their knowledge. Sometimes these simulacra belong to the dead and must therefore represent some portion of those persons that survived death.