لِأَنَّ الْعِلْمَ عِنْدَهُمْ مُقَابِلٌ لِلظَّنِّ
For, in their terminology, knowledge stands in contrast to conjecture.
In one technical usage, ʿilm signifies certainty, whereas ẓann involves a preferred but still nondecisive possibility. Shakk commonly denotes hesitation without preference between opposed possibilities. Exact usage may vary across disciplines.
Saying “it will probably rain” on the basis of clouds can represent conjecture. Grasping a valid mathematical demonstration may exemplify certainty. The fact that some evidence appears credible does not by itself determine whether its conclusion is demonstratively certain; the status depends upon the proof and its conditions.