  In common with other early ancient theories of living things, Democritus seems to have used the term psychê to refer to that distinctive feature of living things that accounts for their ability to perform their life-functions.  According to Aristotle, Democritus regarded the soul as composed of one kind of atom, in particular fire atoms.  This seems to have been because of the association of life with heat, and because spherical fire atoms are readily mobile, and the soul is regarded as causing motion. Democritus seems to have considered thought to be caused by physical movements of atoms also.  This is sometimes taken as evidence that Democritus denied the survival of a personal soul after death, although the reports are not univocal on this.