Philosophers often talk as if it does not make much difference who wrote a piece of philosophy, when, and where, but only whether the arguments it contains are sound. Historians of philosophy should always treat that attitude with suspicion. Philosophical texts about which questions of pseudonymity arise (are they really by the person who claims to have written them?) help to show why, because how they are to be understood is bound up essentially with the question about their authorship and, if they are in fact pseudonymous, what is the purpose behind the apparent deception? The case of the texts attributed to Zera Yacob is a striking example of  where the date and identity of the author matter centrally, whether the texts we have are in fact original, heavily adapted or forged. My talk will try to provide some context. I shall begin by looking at philosophical texts that have been, deliberately or otherwise, attributed to authors who did not write them, such as pseudo-Aristotelian texts, the pseudo-Dionysian corpus, Augustinus Hibernicus, Aethicus Ister, the Epistola Trajani (in John of Salisbury’s Policraticus), the Liber XXIV Philosophorum, and Spinoza’s Tractatus Theologico-Politicus. I shall then focus on one particular comparative example: the famous correspondence of the twelfth-century philosopher, Peter Abelard, and his wife-turned-nun, Heloise (and another set of letters that, more recently, has been claimed as an earlier exchange between the two when they were lovers). Like the Ḥatatā, there has been and remains much debate about the authenticity of these texts, and the parallels and divergences between the two discussions throw light on both.