Samkhya philosophy is often called the sister to yoga philosophy. There are many aspects in common to both philosophies and from a historical perspective, both may have developed in a parallel way. In order to understand yoga philosophy deeply it is often recommended that one understands samkhya philosophy well first, as many ideas used are expanded upon in yoga texts. Some say that samkhya is theory and yoga is the practical element.
Samkhya is said to date back to the time of the sage Kapila and it means number or perfect knowledge in Sanskrit.
Like yoga philosophy, samkhya philosophy talks about purusha or the spiritual part of ourselves and prakriti or nature. Prakrati is influenced by purusha.
Prakrati contains three gunas or qualities. These are sattva (purity), rajas (action) and tamas (inaction). Prakrati pervades all parts of the physical universe.
Kapila argued that the existence of God cannot be proven and that God does not exist, but this notion was contested during later periods of samkhya history by other sages. Yoga philosophy differs in that it specifically adds on the concept of Ishvara or supreme spirit.
Other schools of Indian philosophy such as Buddhism state a similar notion to one expressed in samkhya. That is that ignorance is the root of bondage and suffering. In samkhya and yoga philosophy, the self is eternal and is pure consciousness. As ignorance sets in, the self (purusha) starts to identify itself with the physical body and its constituents. In samkhya philosophy the important constituents are manas (the mind), ahamkara (the ego) and mahat or buddhi (the intellect). If one can become free of this false identification and of material bondage, then as in yoga philosophy one can see oneself as the purusha, the true self and hence reach samadhi or enlightenment.
Samkhya may have taken on the Sanskrit meaning numbers as it is primarily concerned with categories of existence or tattvas. Samkhya consists of 25 tattvas in total. Purusha and Prakriti make up the first two of these. Prakriti remains unmanifested as long as the three gunas are in equilibrium with each other. When disequilibrium starts then evolution of the world from prakriti commences too. Intellect evolves and becomes the third tattva. Ego (ahamkara) and mind are the fourth and fifth tattvas respectively. The five sense organs and the five organs of action are next, followed by the five subtle elements and finally the five gross elements. The purposes of evolution are the liberation and enjoyment of purusha.
One interesting aspect of samkhya is that the intellect, ego and mind make up something called antah-karana or internal organ – an idea which is used similarly in yoga philosophy too.
The jiva or living being is also talked about in samkhya. Jiva occurs when purusha is bound to prakriti.