hidden quantity

The part which deals with Sounds, Inflection, and Word-Formation was written by the junior author, who is also mainly responsible for matters of orthography, hidden quantity, etc., throughout the book;

This is consistent with the fact that an observation requires a measurement and, therefore, between observations, the unobservable ψ evolves in an unobservable way (according to Schrödinger's equation). In connection with this important remark, Bell [142] could state that the fact that a so-called hidden quantity “rather than ψ, is historically called a hidden variable, is a piece of historical silliness.”