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

In plain words: People say that in deep sleep a person sees nothing. Yet the power to see is still there, because the seer can never be destroyed. There is simply nothing separate from him to look at.