A Companion to Analysis: A Second First and First Second Course in Analysis
by Thomas Korner
To this, the mathematician replies that the system constructed in Exercise 14.3.1 has exactly the properties that we wish the integers to have and 'if it looks like a duck, swims like a duck and quacks like a duck then it is a duck'. To this, the philosopher objects that she can construct a clockwork toy that looks like a duck, swims like a duck and quacks like a duck. The mathematician replies that, if all that we want from a duck is that it should look like a duck, swim like a duck and quack like a duck, then, for our purposes, the clockwork toy is a duck.🏁