p208

Correct differential equation for a given phenomenon ?

Notice that even a phenomenon that can be described by a differential equation, there are many differential equations that may be used. For example, we could change the variables. Which is the correct description? Of course, none is wrong. That is, (as you can recall algebraic geometry) what corresponds to a phenomenon is a collection (a set) of differential equations. 

Footnote 40 Popper’s original not available

The author could not find a site that allows us to read the original.

Footnote 41 (categorization) addendum