 In Phaedo or On the Immortality of the Soul, loosely modeled on Plato’s dialogue, Mendelssohn combines a paean to Socrates with an elaboration of the dreadful personal, moral, and political implications if a person’s life is her “highest good.” A best-seller of its time, running through three editions, this “classic of rational psychology,” as Dilthey put it, also contains an argument for the simplicity and immortality of the human soul, explicitly singled out for criticism by Kant in the second edition of the Critique of Pure Reason. Mendelssohn supports the notion that the soul is simple and thus indestructible by noting that certain features of the soul, namely, the unifying character of consciousness and the identity of self-consciousness, cannot be derived from anything composite, whether those composite parts be capable or not of thinking. “We would be able neither to remember nor to reflect nor to compare nor to think, indeed, we would not even be the person who we were a moment ago, if our concepts were divided among many and were not to be encountered somewhere together, combined in the most precise ways they can. We must, therefore, assume at least one substance that combines all concepts of the component parts…. There is, therefore, in our body at least one sole substance that is not extended, not composed, but instead is simple, has a power of presentation, and unites all our concepts, desires, and inclinations in itself” (Phädon (2007), pp. 119f (translation slightly altered)/Gesammelte Schriften, 3/1, pp. 96f). As for the human soul’s fate after death, Mendelssohn appeals to divine goodness and providence, which perhaps explains why, following the publication of the Phaedo, he finds himself needing to revisit the proofs for God’s existence. Playing an important role in Mendelssohn’s development of his views of the human vocation and human immortality in the years prior to the composition of the Phaedo is a 1764–65 debate with Thomas Abbt. On the topic of Thomas Abbt’s role in this regard, his correspondence with Mendelssohn, and Herder’s critical reaction to Mendelssohn’s views, see Pollok 2017 and Pollok 2018b. For an insightful study of Mendelssohn’s anthropology more generally, see Pollok 2010.