because of the degree to which they basically independently invented—well before the existence of christianity, we're talking around 600 BCE or so—the same types of ideas often dismissed as a product of the western enlightenment.
The Charvaka epistemology holds perception as the primary and proper source of knowledge, while inference is held as prone to being either right or wrong and therefore conditional or invalid. Perceptions are of two types, for Charvaka, external and internal. External perception is described as that arising from the interaction of five senses and worldly objects, while internal perception is described by this school as that of inner sense, the mind. Inference is described as deriving a new conclusion and truth from one or more observations and previous truths. To Charvakas, inference is useful but prone to error, as inferred truths can never be without doubt. Inference is good and helpful, it is the validity of inference that is suspect – sometimes in certain cases and often in others.
like, this doesn't exactly resemble modern scientific philosophy, but it's a decent approximation of how scientific law ought to be treated ideally—evidence of a trend supports it as a scientific law, but even a single true1 counterexample disproves it.
and like, the conclusions they draw from it:
Charvakas denied metaphysical concepts like reincarnation, an extracorporeal soul, the efficacy of religious rites, other worlds (heaven and hell), fate and accumulation of merit or demerit through the performance of certain actions. Charvakas also rejected the use of supernatural causes to describe natural phenomena. To them all natural phenomena was produced spontaneously from the inherent nature of things.
The fire is hot, the water cold, refreshing cool the breeze of morn;
By whom came this variety ? from their own nature was it born.
are reasonably sound if interpreted generously! modern thermodynamic theory gives us a more thorough explanation of phenomena like fire than the charvakas could ever have hoped for, but while we haven't stopped digging for deeper truth this is still largely the explanation we have for why the cosmos is the way it is. the fermions have mass, the bosons carry force, time is relativistic. by whom came this variety? from their own nature was it born. model to be updated if we find any particles more fundamental than these
as for their opinions on other spiritual matters, i refer you to this hilariously blunt summary:
There is no world other than this;
There is no heaven and no hell;
The realm of Shiva and like regions,
are fabricated by stupid imposters.
— Sarvasiddhanta Samgraha, Verse 8
now this quote was written by a vedic (which is to say religious) scholar summarizing the charvaka position rather than a charvaka themself, so i dunno if this level of straightforward insult was actually typical of charvaka scholars in the past. but i do know that it would be really funny if it was
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i make a point of saying "true" counterexample here because i think something that can be missed in casual philosophy is that many supposed counterexamples to certain scientific theories are not well-supported. don't mistake this for "take every reported exception seriously," because reported counterexamples should be investigated thoroughly and skepticism is warranted when the weight of evidence lies in favor of existing theory. in the cases where the reported counterexample is rigorous, sound, and true enough to survive this investigation, then it disproves or complicates whatever law it contradicts. (this is a process prone to human bias on the side of both parties, but that is true of all human endeavors)
Charvaka believed that there was nothing wrong with sensual pleasure. Since it is impossible to have pleasure without pain, Charvaka thought that wisdom lay in enjoying pleasure and avoiding pain as far as possible. Unlike many of the Indian philosophies of the time, Charvaka did not believe in austerities or rejecting pleasure out of fear of pain and held such reasoning to be foolish.
The Sarvasiddhanta Samgraha states the Charvaka position on pleasure and hedonism as follows,
The enjoyment of heaven lies in eating delicious food, keeping company of young women, using fine clothes, perfumes, garlands, sandal paste... while moksha is death which is cessation of life-breath... the wise therefore ought not to take pains on account of moksha.
A fool wears himself out by penances and fasts. Chastity and other such ordinances are laid down by clever weaklings.
— Sarvasiddhanta Samgraha, Verses 9-12