I used a similar illustration in one of my Royal
Institution Christmas Lectures in 1991. I said I had reason
to believe that among my audience was a psychic,
clairvoyant individual, capable of influencing events
purely by power of thought. I would try to flush this
individual out. \quotation {Let's first establish,} I said,
\quotation {whether the psychic is in the left half or the
right half of the lecture hall.} I invited everybody to
stand up while my assistant tossed a coin. Everybody on the
left of the hall was asked to \quote {will} the coin to
come down head. Everybody on the right had to will it to be
tails. Obviously one side had to lose, and they were asked
to sit down. Then those who remained were divided into two,
with half \quote {willing} heads and the other half tails.
Again the losers sat down. And so on by successive halvings
until, inevitably. after seven or eight tosses, one
individual was left standing. \quotation {A big round of
applause for our psychic.} He must be psychic, mustn't he,
because he successfully influenced the coin eight times in
a row? 
