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

In plain words: In deep, dreamless sleep a person has no thoughts. Still, the one who thinks is still there and cannot be destroyed. There is simply nothing separate from him to think about.