The concept of measurement plays both a central and a problematic role in quantum mechanics.

First of all, and unusually, the word ‘measurement’ figures in the fundamental axioms of quantum mechanics - in principle (3) of Chapter 2 for example - and in this quantum physics is quite unlike classical physics as a whole. For though a measurement made on a classical system naturally involves an interaction between the system and a measuring apparatus, there is nothing special according to classical physics about measurement interactions. Describing the measuring process is a straightforward problem of applied classical physics. The classical laws of motion do not break down during the measuring process.

For example, suppose you observe a classical particle using light bounced off it. In the classical account you can make the disturbance on the observed system as small as you like and, ‘in principle', you can observe the system with arbitrary accuracy. ‘In principle’ - a favorite expression of the philosopher of physics - means what it always means in physics. The contrast is with ‘in practice'. Of course, in practice there are all sorts of limitations on measurement. There are also some theoretical limitations, like those due to thermodynamics. But these are not the limitations imposed by the theory we are considering, namely classical mechanics. Such limitations as there are are imposed by some other theory. (The expression ‘in principle’ tends to be used when we are discussing the principles of a particular theory and, theoretically but not in practice, physics is a collection of disjoint theories.)