After the Mind the creative energy, next in order, is Purushic Ether, associated with the senses of knowledge, on which the Nyaya system is based. Nyaya and all other systems above it understand it in its true character, and it is only the Sankhya that does so imperfectly.
Vedanta examines the problem in relation to the energy of the Soul, and holds that the senses of knowledge cannot function without the consciousness of the Soul, and that the Soul itself resides in the ether of the Heart. Yoga examines the relation of the senses of knowledge to Buddhi, and of the Sun to Ether; Vaisesika treats of them in relation to the Mind; Nyaya in their own character; while the Sankhya in the light of the senses of action, Prakrti or Food. All these points of view find their place in the Upanishads.
The Senses of Knowledge (Ether) in Vedanta: We have explained that Vedanta examines the problem of the senses of knowledge in relation to the Soul or Prana, characterised by consciousness and action. Hence it is said that breath or Prana alone is the conscious Self, and that the senses of knowledge and action as well as their objects and functions are all portions of this self-consciousness. Then we are told that Ether is supported by breath; that there is Ether within the Heart; and that the intelligent person (purusha or Soul) lies in the Ether, which is in the Heart.
The Senses of Knowledge in Yoga: The idea of the senses of knowledge in Yoga corresponds to that of Buddhi in Nyaya, and this has already been explained.
The Senses of Knowledge in the Vaisesika: The idea of the senses of knowledge in the Vaisesika corresponds to that of the Mind in Nyaya, and that has already been explained.
The Senses of Knowledge in Nyaya: The theory of Nyaya is based on the creative energy of Purushic Ether, associated with the senses of knowledge; and so it examines them in their true character, which includes their relation to the Soul, Buddhi and Mind on the one hand, and the senses of action on the other of these the first three have been examined, and the last refers to the Sankhya. In this connection we have pointed out that the idea of Sacrifice is closely associated with the Nyaya system; and with reference to the senses of knowledge, it implies their control.
The Senses of Knowledge in the Sankhya: The idea of Sankhya is, except for Sacrifice, not far removed from that of Nyaya; and the senses of knowledge are always conceived as twin-born with the senses of action, in the light of which the Sankhya examines their character and so the senses of knowledge are usually associated with those of action in the Upanishads.
Further, we have observed that the Sankhya refers all things to Prakrti, which may be identified with Food or the physical body. Hence the senses of both knowledge and action are spoken of as part of our body.