  Measuring instruments, especially ones that observe continually over time,
have a reality to them that is independent of the states that stimulate their
senses. This is the Principle of Instrument Autonomy. Although the mathematical
concept of an instrument implicitly embodies this principle, the conventional
analysis of continual observation has become overwhelmingly focused on state
evolution rather than on descriptions of instruments themselves. Because of
this, it can be hard to appreciate that an instrument that observes for a
finite amount of time has an evolution of its own, a stochastic evolution that
precedes the Born rule and Schr\"odinger equation of the measured system. In
this article, the two most established of the continually observing
instruments, the Srinivas-Davies photodetector and the Goetsch-Graham-Wiseman
heterodyne detector, are reviewed with an emphasis on the autonomous instrument
evolution they define, made explicit by application of the recently introduced
Kraus-operator distribution function. It is then pointed out how the heterodyne
instrument evolution is a complete alternative to the original idea of energy
quantization, where the usual ideas of \emph{temperature} and \emph{energy} of
a \emph{state} are replaced by the \emph{time} and \emph{positivity} of the
\emph{instrument}.
