  Szilard's one-molecule engine is operated by a Maxwell's demon attempting to
convert heat to work. It is argued here that when using the demon to relate
information to work (Landauer's principle), the demon's goal must be regarded
as an implicit assumption that should not be taken for granted. Other demons
than the standard one can be constructed depending on their assumed goal. When
the demon is purely mechanical, the goal is instilled in it by its design. When
the demon is regarded as "intelligent" (Maxwell) and "endowed with free will"
(Thomson), it is not so clear whence he could acquire a specific goal. A
solution is to subject the demon to evolution by natural selection, which in an
extended form can provide the demon with a genuine goal for which work is
necessary. This implies that the information the demon acquires and uses for
operating the engine becomes functional for the demon, and thereby acquires a
real physical status. Such information, called functional information, applies
to all life and technology. It should be distinguished from the formal,
causally ineffective information that can be defined, more arbitrarily, for any
system. Information in general is not physical, only functional information is.
A similar functional/formal distinction applies to close associates of
information, in particular entropy, probability, and the second law of
thermodynamics.
