The contributions of Peirce’s scientific work to his philosophy have not heretofore been noticed. His psychological experiments extended the realm of admitted observation: by making putative observations, finding that they agree and that their agreement can be explained by what is putatively observed, what was putative is verified. There is no empirical knowledge without metaphysical but fallible presuppositions; they can only be confirmed by the success of the observations they permit. Peirce expanded empiricism in two directions, both surprising. His astronomical work, ranking stars by order of magnitude, required careful attention to one’s own sensations: interpersonal agreement in that ranking suggested the possibility of phenomenology. And the impressionistic ranking of stars’ magnitudes became the model for Peirce’s idea of normative science. The seemingly oddest of his studies, the ’Great Men Study’, provided evidence that normative impressions, as they agree, are observations. It remained only to make the presumed metaphysics intelligible.