Two Versions of Practical Reason

Hume and Thomas Aquinas

Abstract

Thomas Aquinas would have agreed with Hume's approach in the “is-ought question”, also called "Hume's law", because both would agree that it is not theoretical reason that encourages moral action. To act right is characteristic of the common man, not of metaphysicists. The “is-ought question” opens up a very interesting field of research on the difference between theoretical reason and practical reason, as well as on the required functions of the latter and the type of causality it exerts on our will. The problem of the use of practical reason as a tool is also raised, either because it is considered as a mere ancillary reason —slave to the passions, as in Hume’s case— or because of its degradation due to the disarray of the passions —as in the case of Thomas Aquinas. This investigation also leads us to value a philosophy that distinguishes different causalities in the realization of moral action, as opposed to a different philosophy that reduces the causality of such action only to efficient cause, therefore ignoring the spirituality of mankind.