It's just a thermodynamics analogy.
Classical economists write theories from thin air and then try to fit them to observations instead of an observation-first approach like the natural sciences.
The form of the theory is almost always calqued from somewhere else, and that somewhere else is almost always thermodynamics because involves statistics and lets you do that micro to macro bullshit.
So they took the ideal gas law, PV = nRT, and made their theory of money
PQ = MV
Pressure -> Average prices
Volume -> Real output
Number of particles -> quantity of money
Gas constant R -> lmao we don't need physical units, we're economistsPost too long. Click here to view the full text.