When we measure the properties of
a system, we are measuring an average taken over the many microstates the system
can occupy under the conditions of the experiment. The concept of the number of
microstates makes quantitative the ill-defined qualitative concepts of ‘disorder’ and
‘the dispersal of matter and energy’ that are used widely to introduce the concept of
entropy: a more ‘disorderly’ distribution of energy and matter corresponds to a
greater number of microstates associated with the same total energy.