What a strange condition to be visited by your perfect double, your doppelganger. Dr. Oliver Sacks, in his new book on hallucinations, calls these episodes "autoscopic doubles," and he cites a number of cases from medical history.

Luckily, autoscopic doubles obey certain rules. "The autoscopic double is literally a mirror image of oneself, with right transposed to left and vice versa, mirroring one's positions and actions," says Sacks.

So when Linnaeus hallucinated and a second, phantom Carl Linnaeus would appear, like when he was in his garden, checking out a plant or plucking a flower, and he could see, at a respectful distance, the Other Carl stooping and plucking the same way at the same time, Linnaeus didn't fear his phantom; in fact he got used to it.

When You're Visited By A Copy Of Yourself, Stay Calm by Charles Michelet and Robert Krulwich, NPR