In the 30 years following its fullest presentation in 1824, the mathematical psychology of J. F. Herbart (1776-1841) was widely taught and often favorably discussed by scholars such as M. W. Drobisch. However, Herbart's theory received considerable criticism after the introduction of experimentation into psychology by Fechner and Wundt; Ebbinghaus was more friendly toward it. In the period from about 1885 to 1910, with the focus on unconscious processing in Freud's theory and the revival of interest in Herbart's educational theory, one might have expected interest in Herbart's mathematical psychology to rekindle, but this was not the case. The theory remained neglected throughout the 20th century, but it deserves reappraisal at the start of the 21st century.In an earlier article, we (Boudewijnse, Murray, & Bandomir, 1999) focused on the mathematical aspects of the theory of mental events propounded by Johann Friedrich Herbart (1776-1841) in Part 1 of Psychologic als Wissenschaft (Psychology as Science; Herbart, 1824/1890). We did not claim to summarize all of