  Landauer's principle, often regarded as the foundation of the thermodynamics
of information processing, holds that any logically irreversible manipulation
of information, such as the erasure of a bit or the merging of two computation
paths, must be accompanied by a corresponding entropy increase in
non-information bearing degrees of freedom of the information processing
apparatus or its environment. Conversely, it is generally accepted that any
logically reversible transformation of information can in principle be
accomplished by an appropriate physical mechanism operating in a
thermodynamically reversible fashion. These notions have sometimes been
criticized either as being false, or as being trivial and obvious, and
therefore unhelpful for purposes such as explaining why Maxwell's Demon cannot
violate the Second Law of thermodynamics. Here I attempt to refute some of the
arguments against Landauer's principle, while arguing that although in a sense
it is indeed a trivial and obvious restatement of the Second Law, it still has
considerable pedagogic and explanatory power, especially in the context of
other influential ideas in 19'th and 20'th century physics. Similar arguments
have been given by Jeffrey Bub [arXiv:quant-ph/0203017].
