The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
11. If it be said that, in consequence of the ill-foundedness of reasoning, we must frame our conclusions otherwise; (we reply that) thus also there would result non-release.
In matters to be known from Scripture mere reasoning is not to be relied on for the following reason also. As the thoughts of man are altogether unfettered, reasoning which disregards the holy texts and rests on individual opinion only has no proper foundation. We see how arguments, which some clever men had excogitated with great pains, are shown, by people still more ingenious, to be fallacious, and how the arguments of the latter again are refuted in their turn
by other men; so that, on account of the diversity of men's opinions, it is impossible to accept mere reasoning as having a sure foundation. Nor can we get over this difficulty by accepting as well-founded the reasoning of some person of recognised mental eminence, may he now be Kapila or anybody else; since we observe that even men of the most undoubted mental eminence, such as Kapila, Kanâda, and other founders of philosophical schools, have contradicted one another.
But (our adversary may here be supposed to say), we will fashion our reasoning otherwise, i.e. in such a manner as not to lay it open to the charge of having no proper foundation. You cannot, after all, maintain that no reasoning whatever is well-founded; for you yourself can found your assertion that reasoning has no foundation on reasoning only; your assumption being that because some arguments are seen to be devoid of foundation other arguments as belonging to the same class are likewise devoid of foundation. Moreover, if all reasoning were unfounded, the whole course of practical human life would have to come to an end. For we see that men act, with a view to obtaining pleasure and avoiding pain in the future time, on the assumption that the past, the present, and the future are uniform.--Further, in the case of passages of Scripture (apparently) contradicting each other, the ascertainment of the real sense, which depends on a preliminary refutation of the apparent sense, can be effected only by an accurate definition of the meaning of sentences, and that involves a process of reasoning. Thus Manu also expresses himself: 'Perception, inference, and the sâstra according to the various traditions, this triad is to be known well by one desiring clearness in regard to right.--He who applies reasoning not contradicted by the Veda to the Veda and the (Smriti) doctrine of law, he, and no other, knows the law' (Manu Smriti XII, 105, 106). And that 'want of foundation', to which you object, really constitutes the beauty of reasoning, because it enables us to arrive at unobjectionable arguments by means of the previous refutation of objectionable arguments 1. (No fear that because the
pûrvapaksha is ill-founded the siddhânta should be ill-founded too;) for there is no valid reason to maintain that a man must be stupid because his elder brother was stupid.--For all these reasons the want of foundation cannot be used as an argument against reasoning.
Against this argumentation we remark that thus also there results 'want of release.' For although with regard to some things reasoning is observed to be well founded, with regard to the matter in hand there will result 'want of release,' viz. of the reasoning from this very fault of ill-foundedness. The true nature of the cause of the world on which final emancipation depends cannot, on account of its excessive abstruseness, even be thought of without the help of the holy texts; for, as already remarked, it cannot become the object of perception, because it does not possess qualities such as form and the like, and as it is devoid of characteristic signs, it does not lend itself to inference and the other means of right knowledge.--Or else (if we adopt another explanation of the word 'avimoksha') all those who teach the final release of the soul are agreed that it results from perfect knowledge. Perfect knowledge has the characteristic mark of uniformity, because it depends on accomplished actually existing things; for whatever thing is permanently of one and the same nature is acknowledged to be a true or real thing, and knowledge conversant about such is called perfect knowledge; as, for instance, the knowledge embodied in the proposition, 'fire is hot.' Now, it is clear that in the case of perfect knowledge a mutual conflict of men's opinions is impossible. But that cognitions founded on reasoning do conflict is generally known; for we continually observe that what one logician endeavours to establish as perfect knowledge is demolished by another, who, in his turn, is treated alike by a third. How therefore can knowledge, which is founded on reasoning, and whose object is not something permanently uniform, be perfect knowledge?--Nor can it be said that he who maintains the
pradhâna to be the cause of the world (i.e. the Sânkhya) is the best of all reasoners, and accepted as such by all philosophers; which would enable us to accept his opinion as perfect knowledge.--Nor can we collect at a given moment and on a given spot all the logicians of the past, present, and future time, so as to settle (by their agreement) that their opinion regarding some uniform object is to be considered perfect knowledge. The Veda, on the other hand, which is eternal and the source of knowledge, may be allowed to have for its object firmly established things, and hence the perfection of that knowledge which is founded on the Veda cannot be denied by any of the logicians of the past, present, or future. We have thus established the perfection of this our knowledge which reposes on the Upanishads, and as apart from it perfect knowledge is impossible, its disregard would lead to 'absence of final release' of the transmigrating souls. Our final position therefore is, that on the ground of Scripture and of reasoning subordinate to Scripture, the intelligent Brahman is to be considered the cause and substance of the world.
315:1 The whole style of argumentation of the Mîmâmsâ would be p. 216 impossible, if all reasoning were sound; for then no pûrvapaksha view could be maintained.