'And when (it is said that) there (in the Sushupti) he does not know, yet he is knowing, though he does not know. For knowing is inseparable from the knower, because it cannot perish. But there is then no second, nothing else different from him that he could know.

In plain words: In deep, dreamless sleep a person does not know any object. Still, the knower is there and cannot perish. There is simply nothing separate from him to know.