English Moral Philosopher, Professor of Moral Philosophy at the University of Oxford and University of Florida
Our argument ... will result, not upon logic by itself--though without logic we should never have got to this point--but upon the fortunate contingent fact that people who would take this logically possible view, after they had really imagined themselves in the other man's position, are extremely rare.
I have been maintaining that the meaning of the word 'ought' and other moral words is such that a person who uses them commits himself thereby to a universal rule. This is the thesis of universalizability.
It is, most fundamentally, because moral judgments are universablizable that we can speak of moral thought as rational (to universalize is to give the reason); and their prescriptivity is very intimately connected with our freedom to form our own moral opinions (only those free to think and act need a prescriptive language).