Ernst Mach Quote

Thought experiment is in any case a necessary precondition for physical experiment. Every experimenter and inventor must have the planned arrangement in his head before translating it into fact.

'On Thought Experiments' (1897), in Erwin H. Hiebert (ed.), Erkenntnis und Irrtum (1905), trans. Thomas J. McCormack and Paul Foulkes (1976)