According to Vedic principles, bones and dung are generally considered very impure. If one touches a bone or stool, he must take a bath immediately. That is the Vedic injunction. Yet the Vedas also enjoin that a conchshell, although the bone of an animal, and cow dung, although the stool of an animal, are very sanctified. Even though such statements appear contradictory, on the basis of the Vedic version we still accept the fact that conchshells and cow dung are pure and sanctified.
Out of four main types of evidence-direct perception, hypothesis, historical reference and the Vedas-Vedic evidence is accepted as the foremost. If we want to interpret the Vedic version, we must imagine an interpretation according to what we want to do. First of all, we set forth such an interpretation as a suggestion or hypothesis. As such, it is not actually true, and the self-evident proof is lost.
The Bhagavad-gītā is also within the Mahābhārata;therefore all the statements of the Bhagavad-gītā are self-evident. There is no need for interpretation, and if we do interpret, the entire authority of the Vedic literature is lost.
Ref >> Srila Prabhupada Vani.