Based on observation (i.e., empirical knowledge), the reverse of a priori. Used in mathematics and logic to denote something that is known after a proof has been carried out. In philosophy, used to denote something that can be known from empirical experience .
Thus, "blank slate". Romans used to write on wax-covered wooden tablets, which were erased by scraping with the flat end of the stylus. John Locke used the term to describe the human mind at birth, before it had acquired any knowledge .
Said by Julius Caesar upon crossing the Rubicon in 49 BC, according to Suetonius. The original meaning was roughly equivalent to the English phrase "the game is afoot", but its modern meaning, like that of the phrase "crossing the Rubicon", denotes passing the point of no return on a momentous decision and entering into a risky endeavor where the outcome is left to chance .