 It is useful to distinguish three grades of descriptive adequacy of a definition: extensional, intensional, and sense. A definition is extensionally adequate iff there are no actual counterexamples to it; it is intensionally adequate iff there are no possible counterexamples to it; and it is sense adequate (or analytic) iff it endows the defined term with the right sense. (The last grade of adequacy itself subdivides into different notions, for “sense” can be spelled out in several different ways.) The definition “Water is H2O,” for example, is intensionally adequate because the identity of water and H2O is necessary (assuming the Kripke-Putnam view about the rigidity of natural-kind terms); the definition is therefore extensionally adequate also. But it is not sense-adequate, for the sense of ‘water’ is not at all the same as that of ‘H2O’. The definition ‘George Washington is the first President of the United States’ is adequate only extensionally but not in the other two grades, while ‘man is a laughing animal’ fails to be adequate in all three grades. When definitions are put to an epistemological use, intensional adequacy is generally insufficient. For such definitions cannot underwrite the rationality or the aprioricity of a problematic subject matter.