This chapter examines the surprising role a late nineteenth-century controversy over the scientific status of psychology played in scholarship written during that era on the history of philosophy. One side – led by T.H. Green and other erstwhile students of the Plato scholar Benjamin Jowett – contended that the history of philosophy shows that philosophical assumptions underpinning the purported science of mind dead-end in scepticism. Defenders of naturalistic psychology – including G.H. Lewes, T.H. Huxley, E.B. Titchener and later Norman Kemp Smith – looked to admired historical figures of their own to substantiate the view that philosophical reflection must be intimately connected with a properly empirical science of mind. The upshot is that the controversy over psychology’s viability as a natural science was partly fought on the battlefield of historical interpretation. This battle has an under-appreciated legacy: Our contemporary concept of empiricism – including the idea of a British tradition whose central figures are Locke, Berkeley and Hume – is in part a by-product of this now-forgotten fight over psychology. In making his case, the author offers a new analytical framework for tracing historical-theoretical concepts (like empiricism) through time. These concepts are bicephalous in that they typically pick out both a canonical set of authors and a shared set of ideas that are supposed to tie the authors into a single tradition. The author’s new framework is designed to help investigate the evolution of tradition-concepts generally, and he puts it to work in his examination of how the concept of empiricism evolved.