Comment by noelwelsh

> It seems that the notion of side effects must be understood relatively to a predefined system, just like in physics. One wouldn't count heat dissipation or power consumption as a side effect of such a program, although side-channel-attackers have a word to say about this.

Absolutely! When you really dig into it, the concept of an effect is quite ill-defined. It comes down to whatever some observer considers important. For example, from the point of view of substitution quick sort and bubble sort are equivalent but most people would argue that they are very much not.

The preface of https://www.proquest.com/openview/32fcc8064e57c82a696956000b... is quite interesting.