 There are propositions we can only confirm or justify by observation and experiment. There are, for example, various ways to establish that the Earth is round, an especially conclusive one being circumnavigation. There are other propositions that no observation can confirm, nor, indeed, disconfirm; for instance, the proposition that bachelors are unmarried. Since “bachelor” is defined as man who has never been married, we derive this proposition prior to any consultation of the external world. It is a priori.