An enquiry concerning the principles of natural knowledge

by Alfred North Whitehead

10.4 The general reason for this conclusion was given by Einstein in a theorem of the highest importance. He proved that the Lorentzian formulae of transformation from one consentient set to another of the Newtonian group—from set

to set

—are the necessary and sufficient conditions that motion with the one particular velocity

(the velocity of light in vacuo) in one of the sets,

or