Cartesian doubt

Imagine a neurosurgeon whose expertise on the human brain and whose knowledge of daily events are such that he can, with probes, dictate a subject’s experiences. after he has implanted electrodes in the brain of a certain male volunteer, the surgeon causes him to experience the removal of the probes, although they are still in place; then to experience going home through the rain, spending the night with his wife, receiving a call from the surgeon in the morning asking him to return to the laboratory, and returning—all this while he is, in fact, still on the operating table.
The next day, the surgeon does actually remove the electrodes and sends the subject home, whereupon his wife inquires indignantly, “where were you last night?” “Right here with you,” the man replies. “Oh, no, you weren’t,” she rejoins, “and I can prove it. I had the whole neighborhood out searching for you.”
Then the enlightened husband smiles and says, “ah, now I see. that surgeon fooled me. He made me think I came home. but I was on the operating table the whole time.” His smile quickly fades, however, never to return, because from that point forward the poor fellow can never be certain he is still not on the operating table.

—Charles L. Stevenson