Conjure Wife by Fritz Leiber

For a third time their exchange of conversation had come back to the same question. He had the maddening sensation of following a robot that was walking in a huge endless circle and always treading on precisely the same blades of grass as it retraced its path.

With the hopeless conviction that he would not get any further this time, he asked the question again: "But how can you lack consciousness, and at the same time _know_ that you lack consciousness? If your mind is blank, you cannot at the same time be aware that your mind is blank."