Surah 50. Qaf, Ayah 5

Since they reject a priori all thought of life after death, they are perplexed by the lack of any answer to the "why" and "what for" of man's life, by the evident inequality of human destinies, and by what appears to them as a senseless, blind cruelty of nature: problems which can be resolved only against the background of a belief in a continuation of life after bodily "death" and, hence, in the existence of a purpose and a plan underlying all creation.

 Meaning, they are confused about the Prophet (ﷺ): some say he is a poet, some say he is a fabricator, while others say he is insane. They also rejected the Quran as poetry, magic, or ancient fables.

If they deny what has been made clear to them, their minds must necessarily get into confusion. All nature declares the glory and goodness of Allah. Revelation explains the inequalities of this life and how they will be redressed in the Hereafter. If they do not accept this, they are not in a logical position. They cannot reconcile the known with the unknown.