Imam Ibnul-Qayyim Al-Jawzi (rahimahullaah) said:
And using the intellect in opposition to the evidences is the basis of all evil in the world. And it opposes the call of the Messengers (‘alaihim u salaam) in every way. This is because they (the Messengers) called toward putting forward the revelation over one’s opinions and intellects, and those who opposed them did opposite to this. The followers of the Messenger put the revelation over the opinions and intellect, and the followers of Iblees (Satan; may Allaah curse him) – or a subsitute from his substitutes – put the intellect over the evidences.
Muhammad Ibn Abdul-Kareem Ash-Sharastani (rahimahullaah) said in his book Al Milal Wa Nihal:
Know that the first doubt that was put forward was the doubt of Iblees, and the basis of this was putting opinion before an evidence, and in his choosing desire over obedience. And his pride because of the material he was created from – which was fire – over the material which Adam (‘alaihi salaam) was created from – clay. And from this, many other doubts came thereafter.
[From the book: The Summary of Sawaiq Al-Mursalah]