Surah 70. Al-Ma'arij, Ayah 42

I.e., their philosophising about a supposedly "uncreated" world and a hypothetical "self-generation" of life, as well as their blatant "denial", unsupported by any factual evidence, of a life after death or even of the existence of God.

Their talk, their scepticism, is vain, because all spiritual evidence is against it; it is like the foolish play of people who do not think seriously. But the tremendous Day of Judgment and Reality will come, as described in the next two verses.