This paper argues that an unwarranted bias against the international legal personality of non-State entities has been passed down through generations of international lawyers since the 19th century and continues to influence the interpretation and application of contemporary norms across the board in the international legal system. Contrary to conventional wisdom, international law did not apply to States alone until World War II, only to transform during the second half of the 20th century so as to include non-State actors as its subjects. Rather, positive law developments since the 18th century consistently corroborate the notion articulated by Hans Kelsen that the international legal personality of an entity – be it a State, an armed opposition group, or an individual – is solely contingent upon interpretation of international norms. In other words, the answer to the question of the international legal rights and obligations of any entity is, and always was, strictly empirical. Yet, the ‘States-only’ conception of international legal personality is seemingly so ingrained in the minds of many contemporary international lawyers that they inadvertently rely on it when faced with concrete treaty provisions directed at non-State entities. The paper presents two manifestations of this phenomenon: 1) the general perception that EU law is ‘a new legal order’; and 2) the widespread notion that armed opposition groups are only bound by common article 3 of the Geneva Conventions and Additional Protocol II to the extent that these norms reflect customary international law. Similar examples of the ‘States only’ conception’s continued impact may be found in other subfields such as international criminal law, international human rights law, and international investment law. Against this background, the paper questions not only the way in which students worldwide are taught about international legal personality, but the overall value of this concept in international law.