This document defines and discusses the concept of aptitude. It begins by defining aptitude as a natural ability to quickly acquire knowledge or skills. It then discusses different types of aptitudes, including manual, mechanical, and clerical aptitudes. The document also addresses individual differences and variability in aptitudes, noting that two equally intelligent people may differ in their aptitudes for specific tasks and therefore differ in their potential for success. Aptitudes are highly individualized and impact how quickly someone can learn and master new skills or knowledge in a given domain.