Dear Shrinivasa and Shyam,

I beg to differ - tarka is not equivalent to logos.

Though both can mean something like "reasoning," tarka is also sometimes used
negatively to mean approximately "sophistry," and it has none of the theological/anthropological
connotations with which logos is packed.

I do not in fact think that one will find a Sanskrit "equivalent" to logos. A number of terms,
however, do similarly come to be loaded with cultural baggage spanning conceptions of
reason and divinity, though in rather different ways. These include a number of words
for "speech" -- notably śabda and vāc -- and, in Buddhist usage at least, pramāṇa, once that
concept comes to be identified with the nature of buddhahood, as it does in Dignāga and, especially,
Dharmakīrti.

There was an interesting project sponsored by the Indian Council for Philosophical Research about
twenty years ago entitled Samvāda, if I recall correctly, which sought to address related issues by bringing
contemporary philosophers and traditional Sanskrit pandits together in dialogue. The term "rationality"
proved to be a real stumbling point and in the end a Sanskrit neologism was proposed, vaicārikatā (from
vicāra, "rational investigation"). (The noted Dvaitavedānta scholar Prahladacharya was, I believe, the
innovator here.)

Moral of the story: we do best to avoid facile equivalencies between philosophical terms that
have long and complex histories of usage. Of course, useful translation equivalencies in particular
contexts may be warranted (i.e., there may be some occasions on which tarka and logos can
stand for one another), but in many cases that's the best we can reasonably expect.

best regards,
Matthew