Final answer:
The amount refers to the principle of the law of conservation of matter in a closed or isolated system, meaning that matter and energy are conserved within such systems.
Step-by-step explanation:
The principle that the amount never changes within a closed and isolated system refers to the law of conservation of matter, which states that matter is neither created nor destroyed within such a system. In physics, a closed (or isolated) system is defined by its constant mass and the premise that the net external force is zero, thus resulting in the conservation of total momentum.
This is applicable in various scientific contexts, such as chemical reactions, energy transformations, and thermodynamic processes.
An example of this might be observed in Figure 7.15(a) showing a closed system where all the energy remains within the system after it is initiated. The isolated system is even more strict, disallowing any transfer of energy or matter in or out.