Diogenes Laertius' book Lives of the Philosophers is source for many early Greek philosophers including Protagoras; but this work was compiled over six hundred years after Protagoras' death and consequently not accurate. Plato also wrote about Protagoras, considering him a sophist; Aristotle regarded Protagoras as who first introduced a spiritual principle, as a sober man among the inebriated. Protagoras' doctrines can be divided into three groups: "Man is the measure of all things". Greek philosophers, Gorgias and Hippias were also Sophists.