Psychometrics builds on the fundamental premise that psychological attributes are unobservable and need to be inferred from observable behavior. Consequently, psychometric procedures consist primarily in applying latent variable modeling, which statistically relates latent variables to manifest variables. However, latent variable modeling falls short of providing a theoretically sound definition of psychological attributes. Whereas in a pragmatic interpretation of latent variable modeling latent variables cannot represent psychological attributes at all, a realist interpretation of latent variable modeling implies that latent variables are empty placeholders for unknown attributes. The authors argue that psychological attributes can only be identified if they are defined within the context of substantive formal theory. Building on the structuralist view of scientific theories, they show that any successful application of such a theory necessarily produces specific values for the theoretical terms that are defined within the theory. Therefore, substantive formal theory is both necessary and sufficient for psychological measurement.