Bg 13.5

rsibhir bahudha gitam
chandobhir vividhaih prthak
brahma-sutra-padais caiva
hetumadbhir viniscitaih

Word for word: 
rsibhih — by the wise sages; bahudha — in many ways; gitam — described; chandobhih — by Vedic hymns; vividhaih — various; prthak — variously; brahma-sutra — of the Vedanta; padaih — by the aphorisms; ca — also; eva — certainly; hetu-madbhih — with cause and effect; viniscitaih — certain.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
That knowledge of the field of activities and of the knower of activities is described by various sages in various Vedic writings. It is especially presented in Vedanta-sutra with all reasoning as to cause and effect.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Supreme Personality of Godhead, Krishna, is the highest authority in explaining this knowledge. Still, as a matter of course, learned scholars and standard authorities always give evidence from previous authorities. Krishna is explaining this most controversial point regarding the duality and nonduality of the soul and the Supersoul by referring to a scripture, the Vedanta, which is accepted as authority. First He says, “This is according to different sages.” As far as the sages are concerned, besides Himself, Vyasadeva (the author of the Vedanta-sutra) is a great sage, and in the Vedanta-sutra duality is perfectly explained. And Vyasadeva’s father, Parasara, is also a great sage, and he writes in his books of religiosity, aham tvam ca tathanye.… “we – you, I and the various other living entities – are all transcendental, although in material bodies. Now we are fallen into the ways of the three modes of material nature according to our different karma. As such, some are on higher levels, and some are in the lower nature. The higher and lower natures exist due to ignorance and are being manifested in an infinite number of living entities. But the Supersoul, which is infallible, is uncontaminated by the three qualities of nature and is transcendental.” Similarly, in the original Vedas, a distinction between the soul, the Supersoul and the body is made, especially in the Katha Upanisad. There are many great sages who have explained this, and Parasara is considered principal among them.

The word chandobhih refers to the various Vedic literatures. The Taittiriya Upanisad, for example, which is a branch of the Yajur Veda, describes nature, the living entity and the Supreme Personality of Godhead.

As stated before, ksetra is the field of activities, and there are two kinds of ksetra-jna: the individual living entity and the supreme living entity. As stated in the Taittiriya Upanisad (2.5), brahma puccham pratistha. There is a manifestation of the Supreme Lord’s energy known as anna-maya, dependence upon food for existence. This is a materialistic realization of the Supreme. Then, in prana-maya, after realizing the Supreme Absolute Truth in food, one can realize the Absolute Truth in the living symptoms or life forms. In jnana-maya, realization extends beyond the living symptoms to the point of thinking, feeling and willing. Then there is Brahman realization, called vijnana-maya, in which the living entity’s mind and life symptoms are distinguished from the living entity himself. The next and supreme stage is ananda-maya, realization of the all-blissful nature. Thus there are five stages of Brahman realization, which are called brahma puccham. Out of these, the first three – anna-maya, prana-maya and jnana-maya – involve the fields of activities of the living entities. Transcendental to all these fields of activities is the Supreme Lord, who is called ananda-maya. The Vedanta-sutra also describes the Supreme by saying, ananda-mayo ’bhyasat: the Supreme Personality of Godhead is by nature full of joy. To enjoy His transcendental bliss, He expands into vijnana-maya, prana-maya, jnana-maya and anna-maya. In the field of activities the living entity is considered to be the enjoyer, and different from him is the ananda-maya. That means that if the living entity decides to enjoy in dovetailing himself with the ananda-maya, then he becomes perfect. This is the real picture of the Supreme Lord as the supreme knower of the field, the living entity as the subordinate knower, and the nature of the field of activities. One has to search for this truth in the Vedanta-sutra, or Brahma-sutra.

It is mentioned here that the codes of the Brahma-sutra are very nicely arranged according to cause and effect. Some of the sutras, or aphorisms, are na viyad asruteh (2.3.2), natma sruteh (2.3.18), and parat tu tac-chruteh (2.3.40). The first aphorism indicates the field of activities, the second indicates the living entity, and the third indicates the Supreme Lord, the summum bonum among all the manifestations of various entities.