  In this mainly pedagogical article, we discuss under what circumstances
measurements play a special role in quantum processes. In particular we discuss
the following facts which appear to be a common area of confusion: i) from a
fundamental point of view measurements play no special role whatsoever: all
dynamics that can be generated by measurements can be generated by unitary
processes (for which post-selection is no exception), ii) from a purely
physical point of view, measurements are not "outside" of quantum mechanics,
iii) the only difference between the abilities of measurement-based protocols
and unitary circuits for quantum computing comes from practical (technology
dependent) constraints. We emphasize the importance of distinguishing between
differences that are i) fundamental but without physical import, ii)
fundamental and possess physical import, iii) are not fundamental but have
practical import. We also emphasize the importance of separating theoretical
and experimental elements of measurement, primarily projection and
amplification, which are physically very different. Note that since we are
concerned with facts regarding physical processes, this article has little if
anything to do with interpretations of quantum mechanics.
