The ability to stop at will the fluctuations or modifications of the mind which is acquired through constant practice in a spirit of renunciation is called Yoga. True Yoga is practised with a view to attaining salvation. The stoppage of the fluctuations of the mind or its modifications implies the art of keeping only one idea before the mind’s eye and shutting out all other ideas or thoughts. In an advanced state of practice, it is possible to suspend all ideation. The two important features of Yoga to be noted are (i) that there is the suppression at will of the modifications of the mind and (ii) that it is not casual but has been developed into a habit through constant practice, not for gaining a personal end, but in a spirit of renunciation. If without any effort, independently of any volition there is at any time a quiescence; they imagine that at the time they were not conscious of anything. From physical symptoms, such quiescence looks like sleep. Fainting fit, catalepsy, hysteria, etc. also bring about a similar state of mental inactivity. By the conditions mentioned before, this state cannot, however, be regarded as Yoga. Again, some naturally have, or by practice acquire, the power of stopping the circulation of blood or going without food for long or short periods, none of them is Yoga. Holding up the breath for some time in a particular physical mode or posture is not real Yoga either, because in men capable of performing such feats, the power of concentrating the mind at will on any particular object, is not found as a necessary condition.
In the Yogic concentration, where only a single item of thought is kept in the mind to the exclusion of others, there are stages. When the same item of thought can be kept constant in the mind for some length of time, the Yogic process is known as Dhyana (meditation). When the meditation becomes so deep that forgetting everything, forgetting as it were even one’s own self, the mind is fixed only on the object contemplated upon, such voluntary concentration is called Samadhi (intense concentration). This feature of Samadhi should be understood thoroughly. Ignorant people think that any form of quietness of the mind or trance or loss of conciousness of external objects is Samadhi; but that has nothing to do with Yoga.
There are different kinds of Samadhi depending on objects concentrated upon, viz. Samadhi on gross objects like light, sound, etc., on faculties like Ahamkara (Ego-sense) and on entities like the individual self experienced in the cognitions of ‘I’ and ‘mine’. These are called Savija-samadhi (i.e. supported or assisted by an object). The highest form of Savija-Samadhi is to be absorbed in the thought of Self. i.e. in concentration on pure ‘I’. At first, of course, fixity of mind on an object has to be practised; then it develops into Dhyana. When by practice Dhyana becomes deeper it becomes Samadhi. For instance, to attain Samadhi on pure ‘I’, an idea of pure ‘I’ has to be formed first by ratiocination and a particular mental process; then that idea has to be contemplated upon exclusively and developed into Dhyana. When that deepens, it will lead to complete absorption in pure ‘I’. When only the pure I-sense is present and nothing else, the Yogin is not perturbed even by serious pain. No doubt such experience depends on long and constant practice with wisdom and devotion and it is not possible without renunciation of attachment to all gross objects. When the power of Samadhi is acquired by the mind, one can be wholly absorbed in any object of the category of Grahya (knowable, i.e. phenomenal objects comprehensible by the senses), Grahana (internal and external organs) and Grahita (the receiver, the empirical self). In the early stages of practice, however, devotees are instructed by experienced teachers to take up objects for meditation which would soon bring about a blissful feeling because Dhyana on objects of the senses like light, sound, etc., does not bring about blissful feeling quickly and makes the realisation of subtle concepts like pure ‘I’ or individual self, more remote.
While practising devotion and in some cases spontaneously, people have been known to experience a feeling of blissfulness or an expansive feeling as if one were pervading the whole of space. When devotees get such a feeling as a result of devotional practice, it can be utilised as a support for Dharana (fixity), which in course of time can be developed into Dhyana (meditation). If one occasionally gets such a feeling spontaneously, i.e. wihout any practice, but cannot get it when he desires it, then it is of no particular use for purposes of Yoga. Again, the coming of such a feeling does not necessarily mean that Dharana (fixity of thought), Dhyana (meditation) and Samadhi (intense concentration) have been attained; because even on getting such a blissful feeling or a feeling of pervading space, such minds continue to rove in many directions and are not occupied with only one idea. It cannot, therefore, come within the definition of Yoga. That feeling may be a sort of realisation and if fixity is developed on the feeling itself then it might lead to the practice of Yoga… (taken from Yoga Philosophy of Patanjali with Bhasvati by Swami Hariharananda Aranya)