Not the author of the comment, but I can give an example of how it's used in physics. The first law of thermodynamics states (in a simplified form) that the ene

Not the author of the comment, but I can give an example of how it's used in physics. The first law of thermodynamics states (in a simplified form) that the energy into a system is equal to the energy out. So it's a symmetrical process. This means a lot of times we can calculate half of a thermodynamic process, (i.e the heat flow into a system), and know that, over a long enough period of time, the heat flow out of the system will be equivalent.