In this discussion, you will want to
1. Emphasize that many measures are validated primarily on the basis of theory (e.g., Kohlberg's measure of moral development, Piaget's measures of cognitive development).
2. Emphasize that when variables are not theoretically well- defined, they are hard to measure (e.g., IQ). An article that makes this point very clearly is
Rotter, J. B. (1990). Internal versus external control of reinforcement: A case history
of a variable. American Psychologist, 45, 489-493.