Hawzah News Agency – The commander of the faithful, Imam Ali peace be upon him says:

الشَّكُّ على‏ أربَعِ شُعَبٍ: على التَّمارِي، والهَولِ، والتَّرَدُّدِ، والاستِسلامِ، فَمَن جَعَلَ المِراءَ دَيدَناً لَم يُصبِحْ لَيلُهُ، ومَن هالَهُ ما بَينَ يَدَيهَ نَكَصَ على‏ عَقِبَيهِ، و مَن تَرَدَّدَ في الرَّيبِ وَطِئَتهُ سَنابِكُ الشَّياطينِ، ومَنِ استَسلَمَ لِهَلَكَةِ الدُّنيا والآخِرَةِ هَلَكَ فيهِما.

‘There are four aspects to doubt: unreasonable debating [born out of a desire to doubt everything], fear, wavering and undue submission. The one who always resorts to unreasonable debating never again sees the light after darkness. The one who is afraid of what befalls him [doubting as a result] is always turning back on his heels. The one who wavers between his misgivings is trampled by Satan underfoot, and the one who succumbs to the perils of this world and the next is destroyed in them both.’

Source: Nahj al-Balagha, Saying 31