Customary international law (CIL) is particularly vulnerable to the accusation that it is no more than ‘mere assertion’, a creation of the courts, if not downright fantasy. Yet it is in CIL that one finds the strongest claim to objectivity in international law. It is expressed in the doctrine that one of the elements of CIL is state practice, which represents the ‘objective’ element of CIL. It is thought to supplement the ‘subjective’ or ‘psychological’ element of CIL: opinio juris. This chapter argues that the notion of state practice as a set of ‘material facts’ that should be ‘identified’ and from which customary norms can be ‘induced’ is grounded in obsolete epistemology. The identification of state practice is more adequately described as a selection of what deserves to be counted as state practice. It is argued that the starting point for this selective process is opinio juris. Opinio juris does not come after the fact, as a subjective feeling of obligation that is superadded to a set of otherwise objective facts. Opinio juris is the indispensable conceptual framework without which habits and usages cannot even be ‘seen’ as state practice.