A calculus of the absurd

18.3.4 Invariance

One thing which is very useful to know is not just what changes under a transformation, but what remains the same! This relates to a powerful concept in mathematics - symmetry. A common intuitive notion of symmetry is geometric (e.g. discovering lines of “symmetry”), but this is just a subset of “symmetry” - the nicest definition of symmetry I’ve seen is invariance under change.