The quote "A good stock of examples, as large as possible, is indispensable for a thorough understanding of any concept, and when I want to learn something new, I make it my first job to build one", is attributed to Paul Halmos.
Does anyone know where it's written? Thanks a lot in advance!
See P.Halmos, I Want to be a Mathematician: An Automathography (Springer, 1985), page 63: