Ch 18, Sh 41 continue, Bhagawad Gita, Shankar Bhashya

It has been said that the entire transmigratory state together with its roots, characterized by action, agent and results-consisting of the gunas, sattva, rajas and tamas, and projected by ignorance, is an evil. And this also has been said through the imagery of the Tree in the verse, '৷৷.which has its roots upward' etc. (15.1).
It has been further said that, 'after felling that (Tree), with the strong sword of detachment, thereafter, that State has to be sought for' (15.3-4). And, as to that, since all things consist of the three gunas, there arises the impossibility of the eradication of the cause of worldly existence. Hence, it has to be shown how it can be eradicated. Besides, the purport of the scripture Gita has to be summed up, and it has also to be shown that the import of all the Vedas and the Smrtis, which must be put into practice by those who long for the Goal of human life, is verily this much. Hence begin the verses, 'The duties of the Brahmanas, the Ksatriyas and the Vaisyas৷৷.', etc.
Parantapa, O scorcher of enemies; karmani, the duties; brahmana-ksatriya-visam, of the Brahmanas, the Ksatriyas and the Vaisyas; ca, as also; sudranam, of the Surdras-the Sudras have not been included with the others (in the compound word) because, owing to their having a single birth, [Sudras have no right to be invested with the sacred thread which, in the case of the other three castes, symbolizes a second birth.] they have no right to (the study of) the Vedas; pravibhaktani, have been fully classified, have been prescribed by making distinctions among them; according to what?
gunaihi, according to the gunas;
svabhava-prabhavaih, born from Nature. Nature means the Praktrti of God, His Maya consisting of the three gunas. 'Born from Nature' means 'born of these three gunas. In accordanace with these the duties such as control of the internal organs, etc. of the Brahmanas and others have been classified.
Or (the meaning is): The source of the nature of the Brahmanas is the quality of sattva. Similarly, the source of the nature of the Ksatriyas is rajas, with sattva as a subordinate (quality); the source of the nature of the Vaisyas is rajas, with tamas as the subordinate (quality); the source of the nature of the Sudras is tamas, with rajas as the subordinate (quality); for the natures of the four are seen to be tranquility. lordliness, industriousness and dullness respectively.
Or, svabhava (nature) means the (individual) tendencies of creatures earned in their past lives, which have become manifest in the present life for yielding their own results.
The gunas which have that svabhava as their source (prabhava) are svabhava-prabhavah gunah. Since the manifestation of the gunas cannot logically be uncaused, therefore a specific cause [i.e. the tendencies are the efficient cause, and Nature is the material cause.] has been posited by saying that Nature is the cause.
Thus, the duties such as control of the internal organs etc. have been classified in keeping with the effects of the gunas, sattva, rajas and tamas, which are born of Nature, born of Prakrti.
Objection: Well, are not the duties like controlling the internal organs etc. of the Brahmanas and others classified and enjoined by the scriptures? Why is it said that they are classified according to the gunas sattva etc.?
Reply: This objection is not valid. For, the duties like controlling the internal organs etc. of the Brahmanas and others have been classified even by the scriptures verily in keeping with the specific qualities sattva etc.; certainly, not without reference to the gunas. Hence, though the duties have been divided by the scriptures, they are said to have been classified according to the gunas.