Galileo wrote his Dialogue Concerning the Two Chief World Systems in 1632 as a discussion between three friends Sagredo, Saliati, and Simplicio that took place over 4 days in Venice. At a time when no canons for scientific writing (e.g., APA Publication Manual) were available, this work was not only scientific, but also a literary accomplishment. The two chief world systems were, of course, the Ptolomaic and Copernican. For a psychologist interested in individual differences it may be tempting to compare these world systems to one-factor (i.e., general plus specifics, aka Spearman) and multifactor (i.e., Thurstonian) theories of the structure of cognitive abilities and intelligence. As witnessed by the contributions to this volume, these issues remain debated with considerable fervor today and there are even religious overtones present-opponents of the g factor have been called creationists! The analogy, of course, does not quite hold inasmuch as views about the structure of intelligence are not as existentially important as those of the “chief world systems” and it would be pointless to push comparisons too far, even as a purely literary exercise. Besides, although the heliocentric world system did prevail over the geocentric view, neither an extreme one-factor nor multifactor position are likely to succeed.