Duns Scotus versus Thomas Aquinas on Instrumental Causality

Résumé En

The medieval notion of instrumental cause is not limited to what we call today “instruments” or “tools.” It extends way beyond the realm of technology and includes natural entities, for instance, the accidents by which a substance acts on another substance, sensible species in the air acting on a visual faculty, sacraments, bodily organs, and sometimes creatures with respect to God’s action. In all these cases, instrumental causes, like secondary causes in general, are subordinated to a principal cause and contribute to its action and effects. However, the manner in which they do so makes them different from regular secondary causes, and the specifics are not easy to pinpoint. At the occasion of discussions about creation ex nihilo and sacraments, John Duns Scotus challenges Thomas Aquinas’s theory of instrumental causality. Whereas Aquinas does not strongly distinguish between artifacts and natural agents, and postulates a complex superposition of layers of causation, Scotus offers a novel view that clearly separates artificial instrumentality and natural instrumentality, and in both cases explains causation with great economy. Scotus’s in-depth discussion has far-reaching implications. It completely transforms the understanding of instrumental causality in general.