The various commentators of the Shad Darshan system make references to the Charvakas and the Lokayatas. These two groups seem to have been the Materialists and Naturalists of ancient India. I think that there may have even been a third group termed the "Nihilists" (doctrine that human life is meaningless). I don't know the Sanskrit term for the Nihilists. At any rate, all three groups are vilified by the proponents of the six classical schools in the Shad Darshan system. Did any of these marginal and atheistic groups play a positive role in the development of classical Hinduism?