Mbh 12.238.1a

"'"Vyasa said, 'These, then, are the obligatory acts ordained for Brahmanas. One possessed of knowledge always attains to success by going through (the prescribed) acts. If no doubt arises in respect of acts, then acts done are sure to lead to success. The doubt to which we refer is this: whether acts are obligatory or whether they are optional.[936] As regards this (doubt about the true character of acts, it should be said that), if acts are ordained for man for inducing knowledge (by which alone Brahma or Emancipation is to be attained, even then) they should be regarded as obligatory (and not optional). I shall now discourse on them by the light of inferences and experience. Listen to me.[937] With respect to acts some men say that Exertion is their cause. Others say that Necessity is their cause. Others, again, maintain that Nature is the cause. Some say that acts are the result of both Exertion and Necessity. Some maintain that acts flow from Time, Exertion, and Nature. Some say that of the three (viz., Exertion, Necessity, and Nature), one only (and not the other two) is the cause. Some are of opinion that all the three combined are the cause.[938] Some persons that are engaged in the performance of acts say, with respect to all objects, that they exist, that they do not exist, that they cannot be said to exist, that they cannot be said not to exist, that it is not that they cannot be said to exist, and lastly, that it is not that they cannot be said not to exist. (These then are the diverse views entertained by men). They, however, that are Yogins, behold Brahma to be the universal cause. The men of the Treta, the Dwapara, and the Kali Yugas are inspired with doubts. The men, however, of the Krita Yuga are devoted to penances, possessed of tranquil souls, and observant of righteousness.[939] In that age all men regard the Richs, the Samans, and the Yajuses as identical not withstanding their apparent diversity.