I went searching for a more sympathetic take on the philosophical movement, because most of the sources of good provenance come as quotes in texts seeking to refute their views, and the texts of the movement itself are no longer available to be examined directly and in their entirety. I found a scholarly work on it by an Indian doctoral student in philosophy who set out to treat the topic more sympathetically (and I think he succeeded), which is listed in the Sources on this site.
The author, Bhupender Heera, claims that, "We can have a complete systematic work of the materialists of India if we gather them together in a sympathetic manner." His goal is to show the powerful influence of materialist philosophy on Indian traditional thought throughout Indian history.
As he traces the development of Cārvāka, we find that it has gone through what are described as "steps of evolution" and "logical stages of development" as a movement. The first stage in this evolution was an increasingly radical skepticism.
"In its first stage it was merely a tendency of opposition. It called in question all kinds of knowledge, immediate as well as inference. It denies the authority of even the Vedas. At that period, its name was Bārhaspatya."The reactionary nature of this radical skepticism tells us something important about its place in the chronology of Indian philosophy, according to Heera.
"While tracing the origin and development of Indian materialism Dakshina Ranjan Shastri observes that materialism is preached nowhere as a doctrine of philosophy, except as a reaction against some perverted ideas or practices. The materialists of India namely, Brhaspati and his followers, do not pretend to lay down a constructive system of philosophy of their own. They try to refute the foolish orthodoxy of other schools. Thus, in their opinion, it proves that the system of Brhaspati cannot be the first system. It is rather the last.
It raises objections against all the other schools and presupposes the existence of all other schools thereby."
Quite understandably for skeptics, the ancient Indian skeptics were not convinced of the divine origin of the Vedas just as atheists in Europe and the United States today are not convinced of the divine inspiration of the Quran, the New Testament canon, or the Tanakh. But like contemporary skeptics, they went much farther than merely doubting that the traditional religious texts of their culture were authoritative.
"H.T. Colebrook thought that four characteristic doctrines of the Indian materialism were (1) restriction of the means , proof, and sources of knowledge to perception; (2) ultimate existence of the four elements -- earth, water, fire, and air only; (3) denial of the soul to be other than the body; and (4) consciousness as a product of combination. E.B. Cowell, claiming to base his view on Mādhavācārya's account of Indian materialism, calls it Hindu skepticism at its best and compares it with the Greek school of Pyrho and Sextus Empiricus."
The ancient Indian skeptics doubted even the ability of human beings to ascertain the truth by rational inference. Today's skeptics are not nearly so bold; they are generally quite certain that where the scientific community employs rational inference to determine things like the age of the earth or the distance to unimaginably distant galaxies, it is correct.
While they may not explicitly argue that human reason is coextensive with reality or that the Problem of Induction is not a problem at all, most contemporary skeptics intuitively arrive at the conclusion that the patterns discovered by our minds would apply throughout the history of the universe. The skeptics of ancient India had the good sense to find this a questionable assumption, even though it might be an eminently useful one, as David Hume observed much later.
"The Cārvākas are champions of rational empiricism because for them pratyakṣa is the only pramāna and God is non-existent for them. It is the only system in traditional Indian philosophy which does not believe in anything which is beyond the catchment area of perception. Thus he reduces metaphysical world to physical world. For him there is no knowledge which does not subscribe to our five sense organs. So, consistent with the rational empiricism, the Cārvāka denied all that was super-sensible, and supernatural, i.e. of God, soul (apart from body) and the outer world etc. as it could neither be perceived nor proved on ordinary inference and testimony based on the perceptible (acceptable to the Cārvākas)."
Like Hume, the ancient Indian materialists were skeptical of any source of knowledge but their own experience via the perceptual mechanisms of the senses, and even seemed to be inclined to doubt perceptions to some extent. After all, they believed that those who perceived God with the senses were incorrect in their perceptions based on their a priori assumptions about what is perceptible.
This skepticism is of course not a whole philosophy; though it rests on a competing epistemology, and epistemology alone wouldn't suffice for a person trying to get through life. The epistemology it rests upon is made explicit in the second stage.
"In its second stage...recognition of of perception as a source of knowledge and the theory of the identification of the body with the self, was incorporated into it. In that stage, it took the form of a system of philosophy. However, low its position may be, in the rank of philosophical systems, it can by no means be denied that, at that remote period of Indian history, it was the only system of philosophy, worthy of its name. In that period flourished famous materialists like Ajita Ksakambalin, Kambalasvatara and Purana Kasyapa. In that stage it came to be known as hedonism, which was due, perhaps to the corruption of freedom--social, religious and political--which formed the most important features of this school. Gross sensual pleasure superseded bliss or contemplative joy, and licentiousness replaced liberty. Devils occupied the seats of angels."
Here the author suggests that the seats of the materialist philosophers were the seats of angels, taken by the devils. The philosophical purity which came to be formed after the ideas of other schools were purged in the fire of skepticism is quite admirable. What follows their philosophical purity and simplicity is not so admirable, as Heera admits.
"As a consequence of this impact of corruption and misunderstanding Cārvākism originated. In that stage this school preached 'Eat, drink, and be merry' for tomorrow we may die. The reaction to this extreme form of licentiousness was destructive to the very vitality of this school. From that time this extreme form of materialistic school leaned towards spiritualism. So long it had maintained that the body was the self. In that period, being severely attacked by the spiritualists, it gave up the theory that there was no self apart from the body and tried, gradually, to identify the sense organs and the organ of thought with the self. Before that the materialists had affirmed that inference was not a means of knowledge. But in this stage they accepted at first probability and then even inference, though in a restricted form, as a source of true knowledge. Philosophers like Purandara were the advocates of this form of Indian materialism."
Unlike Buddhism, which had a tradition of asceticism to prevent its rejection of the usual ways of knowing promoted by those who followed the Vedas from resulting in licentiousness, the Cārvāka school had nothing to prevent the perfectly normal human tendency toward selfishness from destroying an altruistic project of seeking truth and sharing it with others. Nor, as we see, did it have the collective discipline to resist the attacks of the spiritualists.
We see here the inevitability of hedonism arising from a radical skepticism which denies any transcendent meaning for the material world. Which is not to say that it is necessarily correct to infer from materialism that our moral obligation is to seek short-term pleasure, but that it is a conclusion we human beings will always try to reach because we are so often addicted to our pleasures and we want to rationalize our addictions rather than abandoning them. And with materialism, there is nothing to provide a brake on our tendency to perform that rationalization.
Unsurprisingly, because we humans very much want such a rationalization, those philosophies which provide an easy one become quite popular. Heera does not hesitate to show us that there were also other contributing factors to why it became popular.
several vestiges show that even in the pre-Buddhistic India proclaimers of purely materialistic doctrines appeared. It must have arisen as a protest against the excessive monkdom of the brāhmana priests. The externals of ritualism which ignored the substance and emphasized the shadow, the idealism is the Upaniṣads unsuited to the commoners, the political and social crises rampant in that age, the exploitation of the masses by the petty rulers, monks and the wealthy class, and the lust and greed and petty dissensions in an unstable society paved the way for the rise of materialism in India on the post Upaniṣadic and pre-Buddhistic age. But materialism in Indian philosophy has never been a force. Born in discontent, it soon died in serious thought. Though the materialistic way of life, the way of enjoying the pleasures of the senses and the flesh is as old as humanity itself and will surely lat as long as humanity lasts, yet materialism as metaphysics has never found favour with the Indian philosopher. Jainism and Buddhism arose immediately and supplied the ethical and spiritual background which ejected materialism."
Understandably, people often begin to question the veracity of traditional truth claims when the institutions and social structures which are currently in place seem to be failing to provide for their good, when the hypocrisy of their religious leaders looks larger than their fidelity to the truth, and when their political leaders pay lip service to religious rituals while exploiting their people for their own selfish gain. Thus the religion which is successful in permeating a society, but no longer is lived out as if it were both true and transformative, creates its own enemies.
Like Buddhism, the Cārvāka school was a reaction against the excesses and corruption and clericalism, a noble effort to struggle against the wrongs of the powerful, and a striving for a simpler philosophy which did not rely on ancient texts of dubious authenticity. And like Buddhism, the Cārvāka school became what is so often the beginning of the end for protest movements: popular.
"In our opinion the Cārvāka system appears to be a very dominant and thought-provoking system which paved the way for correction of many ideological wrongs because it always talked about those things which appealed to the common masses. And this might have been the reason that they were called Lokāyata. Since the views of Cārvākas (Lokāyata) were so practical and convincing, they became popular among the masses. Here it would be worthwhile to quote the observations of Rajakrishna Mukhopadhyaya. He writes that the Lokāyata was a very influential system of ancient India. The fact that the sacrificial rites of the Vedas are held in contempt in the Upaniṣads and in the philosophical works, the disappearance of the supremacy of the Vedic gods, and the atheistic opinion of Kapila, Buddha, and Jaimini, all these are the results of the influence of the Lokāyata. Ajatia, a follower of Lokāyata, contradicts the dualism and pluralism of Kacchayana. He does not make any distinction between the soul and the body. ... Buddha's views against the Vedic sacrifices, the memorizing of the Vedic mantras and fruitless repetition to retain them in memory, the caste system, the authority of the Vedas and the worship of the deities, the magical practices and views against the mortification and ascetic practices have their counterpart in the views of the Lokāyata."
The Cārvāka school which began by attacking its predecessors for their unnecessary beliefs, their reliance on unreliable means of knowing, and their ossified orthodox beliefs, ends by taking up unnecessary beliefs, relying on unreliable means of knowing, and becoming ossified into a school of thought which is accepted as part of the landscape of the Indian philosophy it was revolting against.
"In its fourth stage it came to be at one with the Buddhists and the Jainas in opposing the Vedantists and got the common designation nāstika. A nāstika is one who condemns the Vedas -- nastiko vedanindakah."
At this point, the Cārvāka are a fully recognized part of Indian philosophy, albeit a heterodox part. They are placed within the framework of Indian philosophical traditions rather than being an oppositional force outside of it. This stage is of course the cementing of Cārvāka as a popular movement rather than a protest movement animated by its opposition to the status quo. It is also the end of their school of thought, which Heera dates to 1400 CE.
Heera recognizes that getting to this point was a developmental process rather than an event which exploded atheism and materialism into the world like the birth of a great light, which is how post-Enlightenment rationalists and proponents of Western scientific materialism sometimes present it.
"But all systems of philosophy are the growth of years, may be of centuries. The systems which we possess of the different schools of philosophy, each distinct from the other, are rather the last summing up of what had been growing up among many generations of isolated thinkers and cannot claim to represent the very first attempts at a systematic treatment. A large mass of philosophical thought must have existed in India long before there were any attempts at dividing it into well-defined departments of systematic philosophy or reducing it to writing. But such a growth must have required a great length of time. So it is probable that during that long period the views of one system were discussed in another. During that long period anything could be added and anything left out. Subsequently each system reached the form in which we possess it."
I am less sure than Heera is that absolutely anything could have been added or removed, because my experience has been that philosophical schools are very good at excluding that which conflicts with their axiomatic beliefs. That said, I think viewing the philosophical schools as the result of a lengthy developmental process is the most correct way to view them.
"It is not improbable that the Lokāyata school of philosophy, being developed as the first system of philosophy, raised objections against the views of other schools which were even then mere tendencies and which took shape as systems later on. Thus, although, as mere tendencies almost all philosophical thoughts are contemporaneous, as systems they belong to different ages. The school of Brhaspati is regarded as the weakest school of philosophy in comparison with other schools. The law of evolution or gradual development proves that the earliest school is the weakest and the latest is the strongest. If the materialistic school be the weakest, it is probable that it is the earliest also."
Though I think it's useful to examine philosophical developments from an evolutionary perspective, Heera seems to deeply misunderstand the theory of evolution here. Evolution is not a linear process in which things become gradually stronger and stronger, but rather a recursive process in which things continually re-adapt to their environments as their environments change. And that which cannot adapt dies out, though something very similar will likely arise when the environment has changed in such a way that its features are adaptive once again.
Nonetheless, the distinction he makes between the existence of philosophical thoughts and philosophical schools is I think an important one. It's true, for example, that while there are probably always atheistic materialists somewhere at any given time, the existence and strength of philosophical schools which are promoting atheistic materialism waxes and wanes.
"The Cārvākas have sometimes been compared with the Greek philosophers. The heterodox beliefs attributed to Cārvākas immediately call up similarities between them and the Greek philosophers, Democritus and Epicurus. They also show an affinity to the Roman philosopher-poet, Lucretius. ...there can be little question as to Plato's hostile attitude toward naturalism or his later-day fanaticism with regard to proper ideas. The calumny heaped on Epicurus, successor to Democritus, was scarcely less than that enjoyed by Petronius Arbiter. .. Lucretius received better treatment than his naturalistic predecessors, the worst things said about him being that he may have died of a love potion or committed suicide, having realized, perhaps too late, the hollowness of his philosophy. That he was generally ignored for some five hundred years by "Christian and other barbarian philosophers" indicates the extent of his depravity."
The strength of atheistic views which accept only nature in opposition to and exclusion of the supernatural order has been born, lived, and died in ancient Greece as well. And later, inspired to some degree by the classical Greek philosophers and their commitment to reason taught in medieval Catholic schools, those views were reborn in the Age of Enlightenment in Europe, lived on in Germany in the form of the Four Horsemen of Atheism, and thrived for a time in the Communist-ruled countries who did their level best to implement Marxism, many of which are now abandoning Communism. Now it seems to be picking up steam in the United States under the influence of the so-called New Atheists.
In all of these cases, atheism and materialism (or naturalism, whether scientific or otherwise) have been minority positions in the world which have not generally been very strong and not for very long. In the end, the fate of materialism and atheism in India is much the same as the fate of any person in the cosmology of its fellow heretics, the Buddhists. It will be born, live, age, and die, only to be reborn again in another form elsewhere amidst the harrowing pains of radical skepticism rediscovered.
This is the Saṃsāra of Cārvāka, the cycle of death and rebirth of a philosophical school which has arisen many times in many places and eventually fallen in those places. Perhaps there is a way for atheism and materialism to become truly enlightened and subsequently escape the cycle of death and rebirth known as Saṃsāra, but I don't know of it. Perhaps the Buddha does.
By Stephen Shephard - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1130661