What concepts are required to define a "system" in physics ? - as in the phrase "In a closed system ... " or "If two systems are isolated from each other ..."? Is "system" always used as a term of common speech, or can it be defined more precisely? For example, in the thread https://www.physicsforums.com/threa...nd-newtons-2nd-law.891171/page-3#post-5616570 we consider the physical system defined by "the rocket" and it seems we have to resort to common speech to clarify the details of exactly what we wish to include in that system. The problem of specifying a "system" would be simple if we could specify it as a set of "objects". It is common to use the word "mass" to indicate an object, but "mass" is a property of an object, not the object itself. We think of an "object" has having some existence that persists in time. An object like a coffee cup has various non-persistent properties. It can have a variable position, velocity etc. and still be considered "the same" object - even though those properties have changed. In Newtonian physics, we could define an object as something that has a persistent total mass, but an object like a rubber ball need not have a fixed distribution of mass. Perhaps a particular object could be defined by a particular time-varying spatial distribution of mass. We can refer to particular kinds of objects such as "an electron", "a particle" etc. Are there physical models that define such objects completely ? - or do physical models give some of the properties of such objects without claiming to completely define the objects themselves?