Bernard Lewis has cited the Greek philosopher Aristotle who, in his discussion of slavery, stated that while Greeks are free by nature, "barbarians" (non-Greeks) are slaves by nature, in that it is in their nature to be more willing to submit to a despotic government. Though Aristotle does not specify any particular races, he argues that people from nations outside Greece are more prone to the burden of slavery than those from Greece. While Aristotle makes remarks about the most natural slaves being those with strong bodies and slave souls (unfit for rule, unintelligent) which would seem to imply a physical basis for discrimination, he also explicitly states that the right kind of souls and bodies do not always go together, implying that the greatest determinate for inferiority and natural slaves versus natural masters is the soul, not the body. This proto-racism is seen as an important precursor to modern racism by classicist Benjamin Isaac.