A logical theory may be tested by its capacity for dealing with puzzles, and it is a wholesome plan, in thinking about logic, to stock the mind with as many puzzles as possible, since these
serve much the same purpose as is served by experiments in physical science.
Die Hierachie der Propositionen ist praktisch nirgendwo erforderlich und hat bloß für die Auflösung von Paradoxien Bedeutung; [...]
Alfred North Whitehead & Bertrand Russell: Principia Mathematica: Vorwort und Einleitungen, Teil V. (Seite 80 der Deutschen Übersetzung, Suhrkamp 2008)
We need you again, Bertie!!
Johan Galtung: Autobiography
[...] as a rough-and-ready indication of what we mean by a "type," we may say that individuals, classes of individuals, relations between individuals, relations between classes, relations of classes to individuals, and so on, are different types.
Bertrand Russell: Introduction to Mathematical Philosophy
Either a thing is true or it isn't.
If it is true, you should believe it.
And if it isn't you shouldn't.