Samkhya, also Sankhya, (Sanskrit: सांख्य) is a school of Indian philosophy, and is one of the six astika or Hindu philosophical schools of India. It is regarded as the oldest of the orthodox philosophical systems in Hinduism. Its philosophy regards the universe as consisting of two eternal realities: purusha and prakrti; it is therefore a strongly dualist and enumerationist philosophy, characterized by a worldview that sees the universe as an evolving mixture of distinct dualities (light/dark, male/female, etc). Historically, the Sankhya school has been closely associated with the Hindu Yoga school of philosophy. The sage Kapila is traditionally considered to be the founder of the Sankhya school, although no historical verification is possible. The definitive text of classical Sankhya is the Sankhya Karika, written by Ishvara Krishna, circa 200 CE.
The metaphysics of Samkhya
Metaphysically, Samkhya maintains a radical duality between spirit/consciousness (Purusha) and matter (Prakrti). All physical events are considered to be manifestations of the evolution of Prakrti, or primal nature (from which all physical bodies are derived). Each sentient being is a Purusha, and is limitless and unrestricted to its body. Bondage arises when the Purusha is misled as to its own identity and confuses itself with the physical body. The spirit is liberated with the realization that it is distinct from and not restricted to physical matter.
The evolution of primal nature is also considered to be purposeful - Prakrti evolves for the spirit in bondage. The spirit itself is only a witness to the evolution. The evolution obeys cause-and-effect relationships, with primal nature itself being the material cause of all physical creation. The cause and effect theory of Sankhya is called Satkaarya-vaada, and holds that nothing can really be created from or destroyed into nothingness - all evolution is simply the transformation of primal nature from one form to another.
All matter has three fundamental gunas or attributes - sattva (creative attribute), rajas (preservationist attribute) and tamas (destructive attribute). The evolution of matter occurs when the relative strengths of the attributes changes. The evolution ceases when the spirit realises that it is distinct from primal nature and thus cannot evolve. This destroys the purpose of evolution, thus stopping Prakrti from evolving for Purusha.
The purushas (souls) are many, conscious and devoid of all qualities. They are the silent spectators of prakrti (matter or nature), which is composed of three gunas (dispositions): satva, rajas and tamas (steadiness, activity and dullness). When the equilibrium of the gunas is disturbed, the world order evolves. This disturbance is due to the proximity of Purusha and prakrti. Liberation (kaivalya), then, consists of the realisation of the difference between the two.
This was a dualistic philosophy. But there are differences between the Samkhya and Western forms of dualism. In the West, the fundamental distinction is between mind and body. In Samkhya, however, it is between the self (purusha) and matter, and the latter incorporates what Westerners would normally refer to as "mind". This means that the Self as the Samkhya understands it is more transcendent than "mind".
Samkhyan cosmology describes how life emerges in the universe; the relationship between Purusha and Prakriti is crucial to Patanjali's yoga system. The evolution of forms at the basis of Samkyha is quite unique. It is the influence of Samkhya that evolution has been discussed in ancient Hindu scriptures, including the Mahabharata and the Yoga Vasishtha.
Sankhya also has a strong cognitive theory built into it; curiously, while consciousness/spirit is considered to be radically different from any physical entities, the mind (manas), ego (ahamkara) and intellect (buddhi) are all considered to be manifestations of Prakrti (physical entity).
There is no philosophical place for a creator God in the Sankhya philosophy; indeed, the concept of God was incorporated into the Sankhya viewpoint only after it became associated with the theistic Yoga system of philosophy.
[Sankhya philosophy (http://www.philo.demon.co.uk/enumerat.htm)]
[How to drink Coffee - Sankhya view (http://www.philo.demon.co.uk/6Coffee.pdf)] - an introduction to the Sankhya theory of cognition, illustrated in the process of drinking coffee.
[Comparison between Indian and Greek cosmology (http://arxiv.org/abs/physics/0303001)]