All systems of Hindu Philosophy have a place for the individual Soul, but they examine its character from different points of view. Vedanta alone based on the correct perception of the energy of the Heart, has a correct insight into the nature of the Soul. Yoga understands it in the light of Buddhi; Vaisesika in the light of the Mind; Nyaya in the light of the senses of knowledge; and Sankhya in the light of Prakti or Food. Again, according to Vedanta, the Soul is characterised by both knowledge and action in equal degree; and so is its energy expressed in terms of breath or Prana, the vehicle of the Soul and the energy of action at one and the same time.
It is this vital breath that enables us to live, to know and to act; and when it passes, the creature dies. Yoga examines the character of the Soul in the light of Buddhi, and from two points of view. In the one where Buddhi is the first manifestation of the Soul and practically identified with it, it is almost the same as that of Vedanta; but in the second the Soul is conceived in the light of Buddhi, and has Buddhi or knowledge for its goal, and Tapas (meditation) the means to attain it. It is characterised by action indeed for God is the major actor in this system, and the idea of the individual Soul follows that of God but a still greater emphasis is laid on knowledge, characteristic of Buddhi, as the final end. In the Vaisesika the Soul is examined in the light of the Mind, analogous, but inferior, to the consciousness of the Heart; and here too, while the Soul engages in necessary action, knowledge, which characterizes the Mind, is the final goal. In the Nyaya it is conceived in the light of the senses and, like the supreme Purusha in this system, regarded as a mere spectator of the drama of life; and here too knowledge and not action is said to be the end. In the Sankhya the Soul is inseparable from Prakrti, and becomes transformed into Jiva characterised by egoism (Ahankara) or the consciousness born of its association with Prakrti. It can make itself free from the bondage of Prakti only when it realizes its separateness from it; but how that can be done, no one can tell. In any case, the goal of life is freedom from Prakrti and action, to be achieved by means of self-realization or knowledge.
All these ideas of the Soul are to be found in the Upanishads; and we are told of "the Self which consists of food, the Self which consists of breath, the Self which consists of mind, the Self which consists of understanding, the Self which consists of bliss"; and this describes the Soul in the light of all the five systems of philosophy Sankhya, Vedanta, Vaisesika, Yoga and Nyaya respectively.
The Soul according to Vedanta: The Soul according to Vedanta is expressed in terms of Heart-energy, or breath or Prana, the energy of action and the vehicle of consciousness. Brahman himself is identified with the individual Soul abiding in the Heart; and the five vital airs, Prana, Vyana, Apana, Samana, and Udana, which constitute the five gates of the Heart, are said to be the five men of Brahman, the door-keepers of the Svarga-world. Indeed, Prana is said to be Brahman himself. It is at his wish that the senses the eye, the ear, the speech, and the mind and breath function; it is he who directs them; and the Soul in each individual and Brahman are the same.
Thus, like the supreme Soul, the individual Soul, according to Vedanta, dwells in the Heart; it is not born and it does not die. It is characterised by knowledge and, at the same time, engages in action and enjoys its fruit. It is higher than the senses, the Mind, and Buddhi, and yet it contains their energy and pervades them all and so we are told that "the knowing (Self) is not born; it does not die; it sprang from nothing, nothing sprang from it. The Ancient is unborn, eternal, and everlasting; he is not killed, though the body is killed. ... The Self, smaller than the small, greater than the great, is hidden in the heart of that creature. ... Though sitting still, he walks far; though lying down, he goes everywhere. ... When he is in union with the body, the senses, and the mind, then the wise people call him the Enjoyer. ... Beyond the senses are the objects, beyond the objects there is the mind, beyond the mind there is the intellect; the great Self is beyond the intellect". Again we are told that the Self dwells in the five elements (earth, water, fire, air, and ether), the Sun, Moon, and the stars, the three Gunas, as well as the five senses (tongue, eye, ear, skin, and nose), and breath, knowledge and vital seed.TM and then it is said that he who acts is called the elemental Self, and he who causes to act by means of the organs is the inner man (antah purusha).
The Soul according to Yoga: The Soul, according to Yoga, is to be understood in the light of Buddhi, and as characterised by knowledge, though partaking of action too. In this connection Yoga is only one degree removed from Vedanta and, for practical purposes, Buddhi and Soul are often identified. Corresponding to this we are told that the Soul consists of Buddhi or understanding, and are asked to meditate on the Sun as the Self; and we have explained that the Sun symbolizes Buddhi.
The Soul according to Vaisesika: The Vaisesika understands the Soul in the light of the Mind, which is analogous, but inferior, to the energy of the Heart, and holds that it partakes of action, but has knowledge for its goal. Thus it is said that the Self consists of the Mind, and we are told that the "person under the form of the mind is within the heart, small like a grain of rice or barley. He is the ruler of all, the lord of all, he rules all this, whatsoever exists".
The Soul according to Nyaya: Nyaya considers the Soul in the light of the element (Purushic) Ether, or the senses of knowledge, and it holds that the Soul is a mere spectator of the work of Prakrti, dissociated from the physical world and so the Upanishads speak of Bhutatman or the elemental Soul, which dwells in the body composed of the elements, subtle and gross, and is like a drop of water on a lotus leaf. He is pure, firm, stable, undefiled, unmoved, free from desire, remaining a spectator, resting in himself. The Jiva and the higher Soul are sometimes compared to two birds, inseparable friends, who cling to the same tree. One of them eats the sweet fruit, the other looks on without eating. The former is an actor, while the latter a mere spectator of life.
The Soul in the Sankhya: We have explained that the Sankhya examines the Soul in the light of the Prakrtic energy of the Heart, identified with Food. It also conceives of it as Jiva, associated with Prakti and characterised by egoism or Ahankara. Thus we are told that in the beginning there was only Darkness (Tamas or Prakrti) Moved by the Highest, it becomes uneven, and then Rajas and Sattva Gunas are born. Then this Sattva being moved, "the essence flowed forth. This is that part of the Self which is entirely intelligent, reflected in man, knowing the body (ksetrajna), attested by his conceiving, willing and believing. ... Because he came to be, he is the Being". It is obvious that this refers to the Soul according to the Sankhya, which "flows" from Tamas or Prakrti.
In this connection we have already referred to Bhutatman or the elemental Soul, and the Soul which consists of Food, out of which vitality is born; and we are asked to meditate on Water or Prakrti as the Soul. This Soul is said to be overcome by the qualities (Gunas) of nature (Prakrti), and becomes bewildered, and sees "not the creator, the holy Lord, abiding within himself. Carried along by the waves of the qualities (Gunas) darkened in imagination, unstable, fickle, crippled, full of desires, vacillating, he enters into belief, believing, "I am he", "this is mine". This, we might observe, is the idea of Ahankara or the individual Soul associated with Prakrti and characterised by egoism, according to the Sankhya system.