Hello to you all
The claim that Mutazilah were "rational" and enlightened people is one of the biggest lies that have propagated especially the last few years.
The reason is simple. As always, people who know little about Islam and its cultural and judicial history start philosphizing about how rational those mutazilah were and how dogmatic fundamentalist were the orthodox sunnis who distroyed them and sent Islam into the dark ages.
Well here is some news to you, Mutazaliah were the most dogmatic and fundamentalist sect that ever came in Islam, as fundamentalist as Khawarij except they were in power and the last ones were not. Mutazilah believe that any one who committs a great sin was an apostate and that he had to re enter Islam. If he didn't he might be killed for apostacy.
Mutazilis were extreme literalists when it comes to religious rulings. There was little place for reason when a text exists on a subject even if it had a weak narration chain. Just look at their rulings on non-muslim rights to understand.
The only part they used reason in it was faith. In theology the mutazilis who knew how weak Hadiths may be and how complex the quranic text is so they thought it was safer from a theological perspective to use reason in analyzing those text because come on dude, we are talking about God not some minor judicial ruling.
Anyway, the great Moroccan philoshepher Muhammad Abid Al-Jabri explained all this with exceptional insight in his series of books "Critique of Arab Reason":