AD Sertillanges was a French Dominican moral philosopher and theologian who worked closely with the Sillon.
Later he lived in Belgium at the Dominican centre, Le Saulchoir, near Tournai. Among his students there were young MD Chenu, Yves Congar and others who would play a great role during the 20th century.
In Sertillanges’s 1916 book La philosophie morale de St Thomas Aquin, we find:
Prudence being the rule of action (recta ratio agibilium), the various roles of prudence are based on the postulates of action.
The first is counsel, which in in ethics represents invention. “Take counsel, in effect, is to seek (or search).”
The second is judgement, which applies to that which has been found consecrates it.
The third, and the most important of all, because it characterises the virtue of prudence with respect to purely intellectual dispositions, is the imperium. We know what an ingenious conception St Thomas makes of this. It forms an immense part not only of his ethics, but his psychology and even his metaphysics of the soul is based on this word. We have emphasised it considerably as one of the most original thomist concepts.
All this can be summarised as follows: The imperium represents the practical reason playing its ultimately practical role, that is to say restraining action itself, in view of imbibing reason, instead of resting with theoretical determinations. (Stefan Gigacz translation)
In three words: Seek, Judge, Act.