Tafsir of Al-Nas 114:5

It is said that their hearts are intended metaphorically. Others have said that Satan enters the chest, which serves as a vestibule, and casts into it whatever he wishes to cast toward the heart, delivering it there. There is no rational barrier to his entry into the interior of a human being, and the transmitted [scriptural] evidence has arrived—as you have heard—making it obligatory to accept and believe it. Among this is that "Satan flows through the son of Adam as blood flows."

Some people have interpreted this figuratively, arguing that the verse does not necessitate actual entry, as the subsequent clarification indicates. Ibn Sina said that the "whisperer" (al-waswas) is the faculty that causes whispering, which is the imaginative faculty according to its usage by the animal soul. Furthermore, its movement is inverse, for the soul directs itself toward abstract principles. When the imaginative faculty occupies itself with matter and its attachments, that faculty withdraws (yakhnus)—and similar to this is the claim that it is the estimative faculty. It assists the intellect with premises, but when the matter reaches the conclusion, it withdraws and begins to whisper and cast doubt.

It is not hidden that interpreting the speech of Allah the Exalted with such [philosophical] concepts is among the evils of the "whispering, withdrawing one" (al-waswas al-khannas). The judge [al-Qadi] mentioned the latter [the philosophical view] by way of analogy, not as a definitive representation or interpretation, based on a favorable opinion of him.

The grammatical position of al-waswas is either as an adjective [describing Allah], or in the nominative or accusative case to denote disparagement and rebuke. It is appropriate for the reader to pause upon one of these two positions—that is, on al-khannas. Regarding the first [adjectival] position, it is stated in al-Kawashee that pausing is not permissible, but al-Tayyibi challenged this, noting that the claim of impermissibility is questionable given that it is a verse ending (fasilah). In al-Kashf, it is stated that if it is an adjective, then the designation of the pause as "good" (hasan) is not accepted, unless it is on the basis that a "good" pause includes its likeness at a specific verse ending.