Abstract

In previous publications, I have historically traced the prevalence and the influence of an argument—an argument which Kant calls the Achilles, the most powerful, of all rationalist demonstrations in the history of ideas. This proof, which ultimately derives from Plato has been repeatedly used and has had a major influence in shaping philosophic discussions since the Hellenic Age. The form of the argument is fairly straightforward: the essential nature of the soul consists in its power of thinking; thought, being immaterial, is unextended, i.e., simple, having no parts; and what is simple is indestructible; a unity; and an identity. I have attempted to map the incidence and force of this demonstration in the seventeenth and eighteenth centuries—from the Cambridge Platonists to Kant —a time when it becomes crucial in questions concerning the immortality of the soul; the “transcendental” condition necessary for the unity of consciousness ; the necessary and sufficient criteria for the establishment of personal or moral identity; and its use as a sometimes hidden or unconscious premise, but often explicit “principle,” of certain metaphysical and epistemological idealist doctrines. Thus, if thought and soul are essentially unextended, it at once becomes problematic how an immaterial soul can know a material, extended, “external” world.