 Now an a priori proposition is not automatically known. If one is ignorant of the meaning of the relevant terms, then one presumably won’t know the a priori proposition that follows from the meaning. Someone might be unaware, for example, that all language is either verse or prose, or that all planets have sufficient mass to assume hydrostatic equilibrium; yet both propositions follow from the definitions of the relevant words (“prose” is defined as language without the metrical structure of verse; and the official definition of “planet” includes a clause requiring hydrostatic equilibrium).