What explains international investment law as we know it? This thesis untangles the intellectual pedigree of investment protection in international law, taking investment law as both a rhetorical activity and a conversation with the international law tradition. The thesis takes legal history as a tool of critique, by examining how the changing material and intellectual contexts informed legal discussions about protection foreign business interests by international law. By explaining the interpretative choices made by investment treaty tribunals, it highlights adaptations of international law's conceptual fabric. Consequently, the thesis highlights the possibilities of legal innovation latent in the dominant legal discourse. In the four substantive chapters, the thesis explains how investment law emerged as a byproduct of major intellectual shifts in international law, and how sub-textual premises have been carried over into investment arbitration from past argumentative contexts. In particular, the thinking about investment protection in international law has been impacted by intellectual evolutions, namely: the pursuit of international law's unity, the emergence of the new 'lex mercatoria', the international claims settlement process, and the individual right of access to justice. Equally, this thesis makes clear how these evolutions contributed to a reification of investment law as a special regime, led to the application of legal concepts beyond their original frame of reference, and reversed the relationship between the ordinary and the extraordinary remedies.