"A system is in equilibrium when the forces constituting it are arranged in such a way as to compensate each other, like the two weights pulling at the arms of a pair of scales"

This quote by Rudolf Arnheim is referring to the principle of stability in a system. Equilibrium is a state of balance in which opposing forces are equal and cancel each other out. The quote uses the example of a pair of scales to illustrate this idea. The 2 weights on the arms of the scales are in equilibrium when they are of equal weight, so that they stabilize each other out. This is the same principle as a system in equilibrium, where the forces that make up the system are set up in such a method that they are equal and cancel each other out. This develops a state of balance and stability in the system. In conclusion, this quote is describing the concept of equilibrium in a system, where opposing forces are equivalent and cancel each other out, producing a state of balance and stability.