Natural born adepts

For this further reason, one should not on the strength of mere logic challenge something that has to be known from the Vedas. For reasoning, that has no Vedic foundation and springs from the mere imagination of persons, lacks conclusiveness. For man’s conjecture has no limits. Thus it is seen that an argument discovered by adepts with great effort is falsified by other adepts; and an argument hit upon by the latter is proved to be hollow by still others. So nobody can rely on any argument as conclusive, for human intellect differs. If, however, the reasoning of somebody having wide fame, say for instance, Kapila or someone else, be relied on under the belief that this must be conclusive, even so it surely remains inconclusive, inasmuch as people, whose greatness is well recognised and who are the initiators of scriptures (or schools of thought) - for instance, Kapila, Kanada, and others - are seen to have divergent views.

(B.S.B. II.i.11)

It is to be noted that the Vedantin states the important ground rule for this challenge to reasoning; it only applies to those items of knowledge that can only be known from the Vedas. His imaginary interlocutor, with the generic title of the opponent demurs but does not take this stricture into account. He says that the excellence of reasoning is its self correction or constant overcoming of the mistakes of the past. In any case reasoning must be resorted to in the study of the proper import of disputed meanings in the Vedas themselves.

The Vedantin admits that these points are valid to a certain degree.

To this (we Vedantins say): “Even so there is no getting away from the defect.” Although reasoning may be noticed to have finality in some contexts, still in the present context it cannot possibly get any immunity from the charge of being inconclusive; for this extremely sublime subject matter, concerned with the reality of the cause of the Universe and leading to the goal of liberation, cannot even be guessed without the help of the Vedas. An we said that It cannot be known either through perception, being devoid of form etc., or through inference etc., being devoid of the grounds of inference etc.

.....That content of knowledge is said to be the most real since it ever remains the same; and in the world, the knowledge of that kind is said to be right knowledge, as for instance, the knowledge about fire that it is hot. This being the case, people should have no divergence when they have true knowledge, whereas the difference among people who knowledge is based on reasoning is well known from their mutual opposition. For it is a patent fact of experience, that when a logician asserts, “This indeed is the true knowledge”, it is upset by somebody else. And what is established by the latter is disproved by still another. How can any knowledge, arising from reasoning, be correct, when its content has no fixity of form?
........It is also not possible to assemble all the logicians of past, present, and future at the same place and time, whereby to arrive at a single idea, having the same form and content, so as to be the right knowledge. But since the Vedas are eternal and a source of knowledge, they can reasonably reveal as their subject-matter something which is (well established and) unchanging; and the knowledge arising from the Upanisads is alone the true knowledge.

Forgive the long citation but sometimes it seems better to take the risk of being boring to give a proper representation of a view. Shankara wrote in the 9th. C. and his aim was to preserve the final authority of the Vedas in matters that were beyond perception. This also included the code of everyday behaviour which was known as the dharmasastras and was based on the Vedas. Lacking the benefit of evolutionary psychology these codes of conduct were able to stagger along from milennia to millenia. Without the Vedas (cognate with vid/vision), there is left the power of reasoning which in dilemmas could lead to rationalising whatever it is you desire. The ingenuity of Western Philosophers and Bio-Ethicists employed on the subject of abortion is not found in the dharmasastras. It has always been regarded as a grave sin and those who represent that attitude as the sole property of Catholics, Orthodox and ‘fundies’ are simply wrong. It is obvious that an individual guided by their own natural light will not automatically see this. There is so to speak a natural law, a dharma, but it is not something that can be reliably discovered by anybody in good faith applying their native reason. Only those who are enlightened and at one with changeless truth can discern dharmic action in any circumstance.

Interestingly this rejection of the discernment of natural law through the unaided use of reason has been stated with emphasis by the Orthodox theologian David Bentley Hart in his typically trenchant manner.
is-ought and nature's laws
What appears to be the convergence between natural law and a particular perspective on ethics is no more than the overshadowing of the fact of Christianity. Nature and reason without the illumination of the Christ event would point in no obvious direction. Some Thomists affect to find this baffling.

Professor Hart also seems to be sceptical about the Five Ways of Thomas Aquinas as being strict proofs of the existence of God. It is said that a salient difference between Catholic and Orthodox Christianity is that the Orthodox missed the Enlightenment class. One does get the impression that Catholic thinkers require proofs and reasons to keep the holy rollers and snake handlers at a distance.

A counter might be that it is only men of good will who are connatural with the good that are capable of discerning the natural law. That brings in an extra-rational element, connaturality and the variability of persons and cultures. It is natural wisdom cf:connaturality and wisdom
but it is doubtful whether it will give a universal clear and authoritative guide for action.