What are the defects (علل) of Prophetic Hadiths, and is it permissible to dismiss authentic Hadiths on the pretext that science contradicts them?

An 'illah (hidden defect) is an imperceptible, impairing reason that affects an isnad (chain of narration) that otherwise meets the conditions of authenticity. It is identified by collecting and examining all available routes of narration. If it becomes clear that a narrator raised a mawquf (وقف — narrator's own statement) to the Prophet, or connected a mursal (مرسل — lacking a Companion in the chain), or made a mistake, or contradicted trustworthy narrators, then it is considered an impairing 'illah.

A hadith sahih (authentic hadith) is one whose isnad is continuous, narrated by a just and precise narrator from one like him, all the way to its end, and is free from shudhudh (شذوذ — anomaly) and an impairing 'illah.

An authentic hadith cannot contradict certain knowledge. Rather, hadiths might contradict theories that have not reached the level of scientific facts. It is not permissible to reject authentic hadiths claiming they contradict science, for in reality, science bears witness to the Qur'an and Sunnah.

What is known by clear reason cannot be contradicted by Divine Law, and authentic transmitted texts are not contradicted by clear rational thought. If you see a hadith that contradicts reason, or deviates from transmitted texts, or opposes established principles, then know that it is fabricated (mawdu').