Raja-yoga refers to the practices codified in Patanjali's Yoga-sutra and is also known as astanga-yoga and classical-yoga. The term raja (from the verb root raj = 'to reign, to illuminate') means 'royal', the inference being that through raja-yoga we can master or become king of ourselves. The compound raja-yoga seems to have been used from around the 11th century or later in attempts to understand the proper relation between the yoga of Patanjali and hatha-yoga (which emerged as a recognisable path from Tantra towards the end of the first millennium C.E.). In this context the term raja is used to position hatha-yoga as a preparation for what was considered to be the higher meditative practices of classical-yoga. The popular medieval hatha-yoga manual Hatha-yoga-pradipika seems to echo this positioning:

as does the opening verse of the Gheranda-samhita:

One of the reasons why the authors of these hatha-yoga manuals may have been inclined to define the relationship between their school and raja-yoga is that the Yoga-sutra of Patanjali is regarded as the classical exposition of Hindu yoga. Tradition identifies Patanjali with the famous grammarian of the second century B.C.E., and also asserts that he was an incarnation of the Lord of serpents, Adisesa or Ananta, who is often represented as the couch on which Lord Visnu reclines. However modern scholars tend to place the Yoga-sutra in the second century C.E., which means that either the text we have now is a later version, or that the grammarian and the yogin are not the same person. The Yoga-sutra has been the subject of many extensive commentaries, the most notable being the Yoga-bhasya of Vyasa (c. 5th century C.E.) and the Tattva-vaisaradi of Vacaspati-misra (c. 9th C.E.), and it remains the focal text for many schools of yoga today.

Purusa and Prakrti
Patanjali's classical formulation of yoga is also one of the six darsanas (from drs, 'to see') or orthodox philosophical schools of Hinduism, the other five being Samkhya, Vedanta, Mimamsa, Vaisesika and Nyaya. The Yoga-darsana bears the implicit influence of Buddhism, especially in its analysis of suffering and the means that are recommended to alleviate it. However it is most closely associated with the Samkhya-darsana, with which it shares many of its central doctrines (so much so that some commentators discuss the two darsanas together under the heading of ‘Samkhya-Yoga’). According to both Samkhya and Yoga, there are two basic and independent categories of being: prakrti and purusa. Prakrti (from the verb root kr = 'to make, to do' + pra = 'forth') is the unconscious but fundamental activity that produces the manifest universe. Purusa is pure consciousness, unchanging and unattached, yet individuated insofar as purusas are thought to be infinite in number.

There is no intrinsic relation between prakrti and purusa, but prakrti is said to exist for the sake of purusa, their conjunction (samyoga) enabling the purusa's recognition of its own true nature as wholly independent of prakrti in all its aspects. The unenlightened purusa fails to recognise itself as such because it identifies with the contents of citta (from cit = 'to perceive, observe, know'), which for Patanjali is the individuated awareness associated with buddhi (intelligence), ahamkara (ego), and the indriyas which are the five senses (vision, hearing, touch, taste and smell) or organs of knowledge (eyes, ears, skin, tongue and nose), and the five organs of action (the voice, hands, feet, and the organs of excretion and reproduction).

In his commentary on the Yoga-sutra, Vacaspati-misra likens the relation between purusa and prakrti to the reflection of the moon in water. Although the moon is not actually present in the water, its reflection gives the impression of it being so. Similarly, even though purusa is never actually entangled in prakrti, the impression of it being so is given by its reflection in the citta that it falsely identifies with. And just as the reflection of the moon illuminates the water, so does purusa illuminate prakrti. However just as disturbances in the water distort the reflection of the moon, so is purusa unable to recognise itself when citta is restless and disturbed due the varying influence of the gunas.

The gunas are the three basic qualities (or constituents) of prakrti. They combine in an infinite variety of ways to determine the characteristics of all things, including mental states: sattva is pure, steady and buoyant; rajas is activity and passion; while tamas is dullness, inertia and ignorance. When citta is predominantly rajasic or tamasic, the individual purusa is unable to recognise itself and so remains identified with the play of gunas. When citta is predominantly sattvic, the reflection of purusa in prakrti is pure and steady, and this makes it possible for purusa to recognise itself as the consciousness illuminating citta. Patanjali defines yoga in just this way, as the cessation of the fluctuations of citta (citta-vrtti-nirodah, I.2), the implication being that when citta is held steady it is possible for the individual purusa to realise its true nature.

The conjunction of purusa and prakrti therefore ends when purusa is able to discriminate between itself and prakrti. For both Samkhya and Yoga this discrimination (viveka) is synonymous with liberation or kaivalya, which can be translated as aloofness, aloneness, or isolation, and refers to the irrevocable separation of the individual purusa from prakrti. Where Samkhya and Yoga differ is in the means recommended to achieve kaivalya. Samkhya favours a kind of jnana-yoga in which the required discrimination (viveka) is developed though reason alone. Patanjali seems to be less confident of the effectiveness of reason alone when confronting an unruly citta, so he gives us a graduated system of spiritual disciplines that aims to restrain the movements of citta through the cultivation of an unwavering concentration (samadhi). When citta is held steady discrimination (viveka) arises naturally, and with it kaivalya and an end to the suffering that accompanies the cycle of birth, death and rebirth (samsara).

Klesas
According to Patanjali, then, all suffering is the result of the conjunction (samyoga) of the seer (drasta or purusa) and the seen (drsya or prakrti) (II.17). Patanjali's analysis of this suffering takes the form of a fivefold classification of the ways in which purusa’s mistaken identification with the contents of the citta manifests. These are referred to as the klesas (from the verb root klis = 'to suffer, torment or distress') or afflictions.

Proceeding from the fundamental affliction of avidya, the klesas describe not only the ways in which this ignorance of our true nature manifests, but also some of the major motivating factors in our daily lives. For instance Utilitarian ethics, which influences the shaping of public policy in many modern democracies, aims to create the 'greatest good for the greatest number', and the 'good' is usually defined as that which maximises pleasure or happiness and/or minimises suffering. If we add to the mix the instinctual and moral drives to preserve life, then we can appreciate the extent to which Patanjali's analysis of the causes of human suffering corresponds to the elements that many of us believe are essential to a 'good life'.

Keeping this in mind, we can also appreciate how Patanjali's remedy for suffering takes us against the tide of everyday life. This is quite deliberate, though, as the ultimate aim of steadying citta is to disentangle purusa from its presumed involvement in the manifestations of prakrti. The first and most subtle evolute of prakrti, and therefore also the core of the citta, is buddhi (from the verb root budh = 'to enlighten, to know') which is discriminative awareness or intelligence. By steadying citta the usual movement of awareness towards the senses is reversed by first restricting and then eliminating all modifications of buddhi.