Tafsir of Al-Hadeed 57:6

"He causes the night to enter into the day and He causes the day to enter into the night" — Its explanation has passed repeatedly.

"And He is All-Knowing of that which is within the breasts" — meaning, excessive in knowledge of the essence of the breasts, that is, of their secrets. This is an elucidation of the encompassing nature of His knowledge, exalted be He, regarding what they conceal of their intentions, following the explanation of His encompassing knowledge of their deeds which they manifest. It is permissible that what is meant by "the essence of the breasts" is the soul itself and its reality, based on the encompassing knowledge of what is within it, for He knows the former [the secrets] with greater reason.