Likewise, by “imagination” they do not mean imagining something that has no external existence. Rather, in their usage, both terms encompass conceiving through estimation what has no external existence and imagining what does have external existence, which is a sound, truthful perception corresponding to reality.

This is because the terms wahm (estimation) and khayāl (imagination) are often used for conceiving something that has no reality outside the mind. Indeed, this is their familiar meaning in Arabic.

Al-Jawharī said:

“Wahimtu fī al-hisāb—with the present tense awhamu and verbal noun wahman—is used when one makes a mistake in calculation or has a lapse of attention. Wahamtu fī al-shay’—with the vowel a on the h, the present tense awhamu, and verbal noun wahman—is used when one’s thought turns to something while one intends something else. Tawahhamtu means ‘I supposed’; awhamtu ghayrī, īhāman means ‘I led someone else to suppose’, and tawahhum has the same meaning. Ittahamtu fulānan bi-kadhā means ‘I suspected so-and-so of such-and-such’; the corresponding noun is tuhamah, with its consonants vocalized. One says awhama fī al-hisāb mi’ah, meaning ‘he omitted a hundred from the calculation’, and awhama fī salātihi rak‘ah, meaning ‘he omitted a unit of his prayer’. One also says ayhama when someone has become subject to suspicion.”

I say: Thus Abū Nasr al-Jawharī records in his renowned Arabic dictionary, al-Sihāh, that the root of this word is used in connection with mistakes, in the sense of error