The Work and Its Setting
Moses Mendelssohn’s Abhandlung von der Unkörperlichkeit der menschlichen Seele (Treatise on the Incorporeality of the Human Soul) is a short philosophical treatise on the immateriality of the soul. It was written in German and published in 1755, when Mendelssohn was in his mid-twenties and still working as a tutor and bookkeeper in Berlin, having come to the city from Dessau as a largely self-educated young Jewish thinker. The treatise was among the earliest works that brought him to notice in the Berlin Enlightenment circle centered on Gotthold Ephraim Lessing, with whom he soon formed a close friendship, and it appeared in the same years as the essays and dialogues through which Mendelssohn established himself as the leading German popularizer of Leibnizian and Wolffian rationalism.
The treatise is best understood as a piece of rational psychology in the tradition of Christian Wolff, who had systematized Leibniz’s doctrines for the German universities. Its question is old and central to metaphysics: whether the thinking subject, the soul, is a simple, indivisible substance that is not composed of parts and therefore cannot perish by dissolution, as bodies do. Mendelssohn’s aim is to argue, from reason alone and without appeal to revelation, that the soul is incorporeal, meaning it is not extended in space and not made of matter, and that this conclusion has consequences for the soul’s immortality and for the moral order of the world.
Premise and Structure of the Argument
The treatise opens by situating its inquiry against the materialist and skeptical tendencies of the period. Mendelssohn is concerned to show that the question of the soul’s nature is not a matter of faith alone but can be settled by demonstration, and he approaches it through the standard distinctions of the Wolffian school: the difference between extended and unextended beings, between composite and simple substances, and between the bodily and the mental.
The argument proceeds in a sequence of steps. First, Mendelssohn establishes what the soul does, namely that it perceives, represents, judges, and is aware of itself as a unity. Second, he argues that these acts require a subject that is one and undivided, since a composite could not hold together a single representation or a single act of judgment across its parts. Third, he considers the bodily hypothesis, the view that thought is some arrangement or motion of matter, and argues that extended matter, however finely divided, is always a plurality of parts, and so cannot be the bearer of a unified consciousness. Fourth, he draws the conclusion that the soul is simple and therefore incorporeal, and he then reflects on what follows for its persistence beyond the body.
Throughout, Mendelssohn writes with the clarity that would later become his trademark. He avoids unnecessary technicality, frames objections in the form they would present to an ordinary reflective reader, and answers them in measured steps rather than with rhetorical flourish. The work reads less as a systematic treatise in the manner of the scholastic manuals than as a carefully paced inquiry that moves the reader toward a single demonstrative end.
The Central Arguments
The core of the treatise is the argument from the unity of consciousness. Mendelssohn points out that when I perceive a complex object, such as a figure or a melody, the perception is one. I do not experience the parts of the object as separate perceptions owned by separate subjects; I experience them as a whole held together in one act of awareness. Any account of the soul must explain this unity, and an account that locates thought in a collection of material parts cannot. The parts of a body lie outside one another in space; a representation of a whole requires a point in which the manifold is gathered together. Mendelssohn argues that this point must be simple, since anything extended would merely reproduce the problem of plurality.
A second line of reasoning concerns the nature of the representing being’s self-consciousness. The soul does not only represent objects; it represents itself as the one representing them, and it recognizes its own identity through change. Mendelssohn treats this self-relation as a further sign that the thinking subject is a unity and not an aggregate. The “I” that judges and remembers cannot be a mere arrangement of parts, because an arrangement lacks the single point of reference that self-consciousness presupposes.
Mendelssohn then confronts the materialist alternative directly. The materialist may say that thought is a motion or configuration of the brain’s fibers, and that the unity of consciousness is only apparent, a product of closely coordinated parts acting together. Mendelssohn’s reply is that coordination among parts still leaves the parts distinct, and that a manifold of distinct motions cannot constitute a single perception of which the manifold is aware as one. The point rests on the principle that a composite can only contain properties that belong to its parts severally, while the unity of perception is not found in any part taken alone. This is the decisive move, and Mendelssohn presses it through a series of reductions and examples designed to show that every bodily model fails to account for the unity it purports to explain.
Immortality and the Moral Order
Having argued for the incorporeality of the soul, Mendelssohn turns to its consequences. Because the soul is simple, it cannot be dissolved into parts, and because it is not composed of parts, it cannot perish in the way that composite bodies decay. Mendelssohn is careful here: he distinguishes the soul’s indestructibility by division from the question of whether God could annihilate it by an act of will. The claim he defends is that the soul’s natural constitution gives it no internal principle of dissolution, so that its continued existence follows from its nature and not from any special divine intervention. This is the sense in which the treatise supports immortality by reason.
The argument is connected to a theme that runs through Mendelssohn’s broader thought, namely the harmony between metaphysical truth and moral life. If the soul persists, then the moral order of the world, in which virtue and vice are finally to be requited, has a basis in the very nature of the thinking being. Mendelssohn presents the immortality of the soul as something that rational reflection can recognize as fitting to the character of a being capable of perfection, and he treats the soul’s destiny as bound up with its capacity for ongoing improvement. The treatise thus links metaphysical demonstration to the practical concern for the human good that would animate his later work on morality and religion.
Method and Place in Mendelssohn’s Work
Methodologically, the treatise exemplifies what Mendelssohn would later defend as the proper use of reason in matters of religion: argument proceeding from clear concepts, with careful attention to objections, and an openness to the conclusion that reason, when rightly used, supports certain truths traditionally held on faith. This commitment to rational demonstration of the soul’s nature and destiny is one reason the work was received warmly within the Enlightenment milieu of Berlin, and it helped establish Mendelssohn’s reputation as a philosopher capable of defending metaphysical positions with lucidity and rigor.
The treatise also foreshadows the mature Mendelssohn in its attention to the soul’s faculties and to the relation between cognition and feeling. Although the work is concentrated on the question of substance and its incorporeality, it shows the interest in the inner life of the mind that would later find fuller expression in his psychological writings and in his aesthetics. Its confidence that reason can speak meaningfully about the soul’s nature set a pattern that Mendelssohn would pursue throughout his career.
Reception and Significance
The treatise was well received by readers who shared the rationalist premises of the Wolffian school, and it contributed to the reputation that made Mendelssohn a central figure among the Berlin Enlightenment writers. It was part of the body of work that led to his prize-winning essay on the evidence of metaphysical truths, a competition that brought him wide attention and placed him alongside Kant in the discussions of the period, though Kant’s critical turn would soon challenge the claims of rational psychology that the treatise defends.
In the longer view, the Treatise on the Incorporeality of the Human Soul is valued as a clear and disciplined example of eighteenth-century rational psychology, and as the work of an author who would become one of the most important figures in German Jewish intellectual life. Readers today approach it both as a document of the metaphysics of its time and as an early statement of the confidence in reason that shaped Mendelssohn’s entire philosophical project. Its arguments from the unity of consciousness remain a point of interest in the history of debates about the mind, and its linkage of the soul’s nature to the moral destiny of the human being reflects the central aims of Mendelssohn’s thought.
