The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
10. If it be said that purposelessness (of conduct would result therefrom), we deny this on account of the dependence (of work) on that (conduct).
That may be; but for what reason should we abandon that meaning which the term 'karana' directly conveys, viz. the meaning 'conduct,' and accept the merely connotative meaning 'remainder of the works?' Conduct, which the text directly mentions, may be supposed to have for its fruit either a good or an evil birth, according as it is enjoined or prohibited, good or evil. Some fruit will have to be allowed to it in any case; for otherwise it would follow that it is purposeless.--This objection is without force 'on account of the dependence on it.' Such works as sacrifices, and the like, depend on conduct in so far as somebody whose conduct is not good is not entitled to perform them. This we know from Smriti-passages, such as the following, 'Him who is devoid of good conduct the Vedas do not purify.'--And also if conduct is considered as subservient to man 1 it will not be purposeless. For when the aggregate of works such as sacrifices, &c.. begins to originate its fruit, the conduct which has reference to the sacrifice will originate there (i.e. in the fruit) some addition.
[paragraph continues] And it is known from Sruti as well as Smriti that work effects everything 1. It is, therefore, the opinion of Kârshnâgini that the remainder of works only--which is connoted by the term 'conduct'--is the cause of the souls entering on new births. For as work may be the cause of new births, it is not proper to assume that conduct is the cause. If a man is able to run away by means of his feet he will surely not creep on his knees.
120:1 I.e. as something which produces in man a samskâra analogous to that produced by other preparatory or purificatory rites such as bathing, &c.--In the preceding sentences conduct had been spoken of not as purushârtha but as karmâṅga. In that case it produces no separate result; while if considered as purushârtha it has a special result of its own.
121:1 A clause added to guard against the assumption--which might be based on the preceding remarks--that conduct is, after all, the cause of the quality of the new birth.