The tabi'iyyūn are a group who mostly discuss the realm of nature, animals, and plants. They also engaged in the science of anatomy (tašrīḥ, dissection) of animal organs. There, by witnessing the unique wisdom of Allah and His artistry that leaves a person in wonder (ḥayra), they were compelled to acknowledge the existence of an All-Powerful and All-Wise Being who knows the purposes and aims of such phenomena down to their finest details.
(However, because they spoke so much about nature, they believed that the temperament (mizāj) has a great influence on the moderation (balance) of animal faculties. For this reason,) they supposed that the rational faculty in humans is also subject to temperament. They claimed that with the change of temperament, the rational faculty also changes, and as a result, it cannot be thought to exist again. For this reason, they fell into the belief that when a person dies, he does not return, and thus they denied the hereafter. They rejected Paradise, Hell, resurrection (ḥashr), re-creation (nashr), the Resurrection (qiyāma), and the reckoning. In their view, there is no reward for worship nor punishment for sin. They went to great extremes in this regard and indulged in passions like animals. Thus, these too are considered zindiqs (heretics). (Munkidh, 96)