Samkhya philosophy has long been regarded as one of the most important and influential schools of Indian thought. In the words of Dr. K. B. Ramakrishna Rao, Samkhya "has been absorbed by every branch of Indian philosophy, and it is difficult to find an Indian thought free from the elements of Samkhya, be it philosophical, religious, or mythological". But Indian thought has been dominated by the philosophy of Advaita Vedanta from the time of the Vedanta teacher Sankara up to the present. Samkhya is now considered to have only historical value, and is practically unknown in the west. In his Theism of Pre-Classical Samkhya, Dr. Rao investigates what could have caused the once popular and influential system of Samkhya philosophy to fall out of favor in India.
Dr. Rao challenges the commonly held belief that the system founded by the rishi Kapila was complete at its inception and remained unchanged throughout its history. He argues that Samkhya went through a process of development over a long period of time and was influenced by the currents and cross-currents of thought occurring in the society. Using a method of historical/critical analysis, he reconstructs Samkhya's historical development, citing examples of Samkhya thought contained in the Upanishads, the Bhagavad Gita, the Mahabarata, and other Hindu literature. He asserts that the philosophy that traditionally had been attributed to Kapila was actually Samkhya at a highly developed stage, and considers Kapila to be a mythological figure.
According to Dr. Rao, in the early stages of its development, there was probably nothing to differentiate Samkhya and Vedanta. There are many examples of Samkhya thought in the early and middle Upanishads, but the word Samkhya is never used, and there is no indication that these ideas were a separate philosophy or school of thought. Rather, they are an undifferentiated part of the whole. Likewise in the Bhagavad Gita, which contains many of the ideas found in classical Samkhya, the philosophy appears as an integral part of the whole, although it is here that the first use of the term Samkhya occurs. It was not until the time of the Epic Samkhya contained in the Mahabarata that a separate Samkhya school appears, but even then Samkhya was apparently held in high esteem by the Vedanta school. So what accounts for the change in attitude between the time of the Epic schools and the time of Samkhya's foremost adversary, Sankara?
Dr. Rao contends that the Samkhya of the Upanishads and the Bhagavad Gita contained a definite theistic element that made it more acceptable to its contemporaries. By the time of the systematic (or classical) Samkhya found in the Samkhya Karika of Isvarakrishna, the philosophy had developed into a highly rational form, leading it to "feel agnostic" about the concept of a deity as an overlord and director of creation, and to assign the creative forces entirely to the objective principle, Prakriti. Dr. Rao also argues that the author of the Karika made mechanical deletions to the philosophy in an effort to simplify and shorten it, but that these mechanical deletions created rational difficulties, which made the philosophy susceptible to Sankara's criticism.
Sankara attacked Samkhya on two grounds: first, on the basis that it lacked scriptural authority, and second, on rational grounds. Dr. Rao does not deal directly with Sankara's criticism on lack of scriptural grounds, but argues that it is not wholly acceptable, because Sankara either missed or ignored the historical perspective. Dr Rao admits that the systematic Samkhya has a number of problems on rational grounds. However he contends that these rational problems were created by the mechanical deletions to the philosophy that were made in the classical formulation. In other words, it was the classical formulation itself that led to Samkhya's demise. Had it not been for the rational difficulties introduced there, Sankara's task of overthrowing Samkhya would have been much more difficult.