Kant condemned the probabilism of Catholic moral theology as a vehicle of self-love and moral self-deception. However, Kant accepted two principles that had become pillars of probabilism in the seventeenth century. The first principle, claiming that in doubt about ownership the position of the possessor of a thing is preferable, had been innovatively applied by theological probabilists to the possession of opinions and beliefs. Kant used the possessor principle for similar purposes. Moreover, Kant endorsed a second principle, according to which uncertainly valid laws cannot obligate, in a radical interpretation that had before been championed by theological probabilists. In radical form, both principles pose serious challenges to any ethics falling short of aprioristic certainty and identification with the moral law through self-legislation. Thus, Kant’s ethics (if successful) solves problems that had been unveiled by theological probabilists c. 1650 -1750. By posing such a challenge, which the idea of autonomy helps to solve, the probabilism of Catholic moral theology belongs to the prehistory of autonomy in the early modern era.