This is the first of three chapters on Kant’s engagement with Stoic philosophy in the 1793 Religion within the Bounds of Reason Alone. It sketches neo-Stoic aspects of German Enlightenment debates about human corruption and progress, focusing on Moses Mendelssohn. It then demonstrates that Kant faces an old Stoic problem in the Religion: how is a commitment to providential natural teleology compatible with the supposition that human beings inevitably corrupt themselves from the first use of reason? Kant refers to this corruption as the ‘radical evil in human nature’. Canonical Stoic accounts of human development bracket the fact of such corruption to trace human development in a life that duly ‘follows nature’ according to Stoic ethical command – for such corruption is, precisely, a perversion of natural principles. But how is such developmentally inevitable corruption compatible with a commitment to the providential natural teleology that underwrites hope for human progress? Kant replies by reworking the canonical Stoic account of human development: radical evil must be part of the story that yet reveals this evil to be unnecessary, as should and may be overcome.