Tafsir of Ash-Shu`ara' 26:115

( I am but a clear warner )

This serves as the cause [for his refusal]. It means: I am nothing but a messenger sent to warn those who are morally responsible and to restrain them from that which does not please Him—Glory be to Him and Exalted—regardless of whether they are of the noble or the lowly. How, then, could it be possible for me to cast away those whom you claim to be the lowly?

The gist of this is: I am restricted to warning those who are morally responsible, and I do not transgress that [duty] to the expulsion of the lowly among them. Or, it means: I have no duty but to warn you with clear proof, which I have already done, and it is not my duty to satisfy some of you by casting away others. The gist of this is: I am restricted to warning you, and I do not transgress that to satisfy your desires.

It has been said: The two clauses combined constitute the answer [to their demand]. Placing the pronoun immediately after the particle of negation indicates that they had alleged that he—peace be upon him—was characterized by two attributes: the first being the following of their whims by casting away the believers so that they might believe, and the second being that he is a clear warner. Thus, he restricted the judgment to the second [attribute] to the exclusion of the first. This, however, is not free from [the need for further] investigation.