In "Ringworld", Nessus, a Puppeteer, explains how his race's cowardice is partly a result of a science experiment (the details of which are not given) that proves the Puppeteers have nothing equivalent to an immortal soul, and therefore death is, for their species, eternal oblivion. As a result, the Puppeteer race is fanatically devoted to its own safety. It is worth noting that this explanation is a form of diplomatic, post hoc rationalizing, as their cowardice is a biologically ingrained trait and not actually a rational choice. It is diplomatic, in that it validates in other species what Puppeteers consider to be irrationally insane bravery, although it is doubtful that the Puppeteers genuinely accept other species' claims to having souls.