We have examined the chief problems of life in the light of the different systems of thought, and now we might examine the character of the other energies, from Buddhi to the senses of action, in the same manner.
As we have observed, Buddhi, according to all systems of Philosophy, is the first manifest energy of life. Vedanta, based on the character of the Soul, and Yoga, based on Buddhi itself, understands its character aright; but the three remaining systems, based on the Mind, the senses of knowledge, and the senses of action or Food, understand it imperfectly and only in part. Buddhi in Vedanta is the first manifest form of the unmanifest energy of the Heart and, for practical purposes, identified with it. Yoga understands it as characterised by certainty of thought, as the highest achievement of the intellect, attained by Tapas or meditation. Vaisesika understands it in the light of the Mind and Nyaya in the light of the senses of knowledge; while the Sankhya examines it in the light of the senses of action as well as of Prakrti or Food and regards it as its own first manifestation. Let us see how far this is borne out by the Upanishads.
Buddhi in Vedanta: As we have explained, Buddhi in Vedanta is the first manifestation of the Soul, and the two are often identified. Further, we have observed that the Sun symbolizes Buddhi. Hence the relation between the Sun and the Soul corresponds to that between Buddhi and the Soul in Vedanta, and would give us the character of Buddhi in that system. Thus we are told that the Sun is the outer (manifest) Self, the inner (unmanifest) Self is breath or Prana; that the Sun is but the eye of the Self. Then Prana or breath (Self) is identified With the Sun; and it is said that "it is Prana who shines as the Sun"; and "what I am that is he (Sun), and what he is, that am I"; and so the Sun is said to be the Self of all that moves and rests.
Buddhi in Yoga: Yoga examines Buddhi in its true character as meditation, Tapas, or the power of thought, and as characterised by the energy of the Sun. In this connection we have already explained how the whole world is said to have been created by means of Tapas; and we are told that meditation is the means by which a man sees the real nature of his Self as well as of Brahman. Again, the Sun, symbolizing Buddhi, is said to be the lord of all creatures, and all beings are dependent on him.
Buddhi in the Vaisesika: The Vaisesika considers Buddhi in the light of the Mind; and the former has the Sun and the latter the Moon for its presiding deity (MM. I) and so we are told in the Satapatha Brahmana that "the full Moon, doubtless, is the same as that burning Sun"; and it is said that when Buddhi desires anything, it is transformed into the Mind; and that when Buddhi lives in the Mind; it is nothing but the Mind.
Buddhi in Nyaya: Nyaya examines Buddhi in the light of Purushic Ether or the senses of knowledge; and so we are told that the Sun (symbolic of Buddhi) exists in Ether, and we have already pointed out the connection between Ether and the senses.
Buddhi in the Sankhya: The Sankhya examines Buddhi in the light of Prakrti, conceived as Water; and so it is said that "in the beginning this (world) was Water. Water produced the true. Now what is the true that is the Aditya (the Sun)".