Tafsir of Al-An'am 6:37

(And they said), that is, the leaders of Quraysh, whose ignorance and misguidance reached a point where they were not satisfied by the signs they witnessed—signs before which even solid mountains would crumble—and did not consider them. (Why), meaning "why not," (has a sign not been sent down upon him from his Lord), meaning one that compels belief. (Say), O Muhammad, (indeed Allah is able to send down a sign) from the compelling signs. (But most of them do not know).

They do not know that the non-sending of such a sign, despite His Glory—Exalted be He—having the power to do so, is because its descent would either uproot the foundation of religious obligation (taklif), which is built upon the principle of free choice, or result in their total extermination, as that is a consequence of rejecting a compelling sign.

It is also possible that they did not demand a compelling sign; the failure to consider the signs already witnessed does not necessarily mean they demanded a different one. It is possible they demanded something other than what was present, out of stubbornness and defiance, and the response with the mention of the "compelling sign" is thus an instance of the "wise man's method" (uslub al-hakim), or a response that encompasses their demand by way of a stronger, more eloquent proof.

The prepositional phrase "from his Lord" (min rabbihi) may be linked to "sent down" (nuzzila) or to an implicit term acting as an adjective for "sign." The reference to His Lordship (rububiyyah) over him—peace and blessings be upon him—is an insinuation intended to mock their stance. Restricting the answer to declaring His power—Exalted be He—to send it down, despite that [power] not being under denial, serves to signal that His non-sending of the sign, despite His ability to do so, is due to profound wisdom that must be understood, though they are heedless of it, as the corrective particle "but" (lakinna) indicates. The Manifest Name [Allah] is invoked to evoke awe, while also signaling the causality.

The object of "they know" is either omitted entirely, meaning they are not people of knowledge, or it is implicit, indicated by the context—meaning they do not know anything. The restriction of this lack of knowledge to "most of them" is because some of them are aware of the reality of the situation, yet they persist in their actions out of arrogance and stubbornness. Ibn Kathir read "yunzila" (send down) with a light vowel (takhfif); the meaning here, as has been said, is the same, as no regard was given to whether the descent is gradual or otherwise.