Physics cannot be regarded as validly based upon empirical data until the waves have been expressed as functions of the colours and other sense-data.
"Mysticism and Logic and Other Essays"
Bertrand Russell
Idealism holds, speaking more positively, that philosophers must submit the conceptions and methods which they employ to a preliminary immanent criticism, in order to determine the limits within which they may be validly applied.
"John Dewey's logical theory"
Delton Thomas Howard
In order that a thing may be done validly, it must be done knowingly.
"Napoleon the Little"
Victor Hugo