There are several laws that many people know verbatim and could quote off of the top of their heads. Take Gay Lussac's (The Pressure) Law for instance:
The pressure of a gas of fixed mass and fixed volume,is directly proportional to the gas's absolute temperature.
Which is the wording that is used in many textbooks and on Wikipedia. However, in many instances, this included, it can be difficult to locate the original paper where the law was defined due to the passage of time. A similar example of this could be Pythagoras' Theorum:
The square of the hypoteneuse is equal to the sum of the squares of the other two sides.
So my question is thus, how should one go about citing these definitions? Should we just assume that everyone is aware of their provenance due to them being universally understood laws in academic circles and thus they can stand on their own axiomically?
If it's relevant, my university conforms to their own variant of Harvard referencing.