THE idea that colleges should choose among applicants on the basis of their “academic ability” appeals to both educators and the public. But “ability” has two distinct meanings, which imply different admissions policies. In one usage academic ability means an existing capacity to do academic work. In the other usage academic ability means a potential capacity to do such work. To say that an applicant "has the ability to do differential calculus,” for example, can mean either that the applicant can already do differential calculus or that the applicant could learn differential calculus given opportunity and motivation. To avoid this ambiguity, psychometricians usually call the ability to learn something an “aptitude” while calling current mastery of a skill or body of knowledge “achievement.”