This study has covered a lot of details in Aquinas’s theories about efficient causation and causal powers. Thus, it might be useful to conclude by reiterating how his theories respond to some of the “big picture” philosophical questions about causation. One major question that arises when thinking about causation is: What is essential to the relationship between a cause and its effect? Put otherwise, what is it that ties certain phenomena together as cause and effect? What unifies disparate cases of causation as instances of causation? As noted in the introduction, many contemporary theories conceive of causation as a logical relationship between events. Attempts to specify this logical relationship are routinely defeated by counterexamples. Aquinas’s theory offers an alternative way of conceiving of the relationship between cause and effect. Instead of construing causation as a logical relationship, he sees causation as an ontological phenomenon. As we have seen, causes influence the being of their effects and effects depend on their causes for their existence. The unique way in which efficient causes influence the being of their effects is by action, namely an exercise of power. Aquinas recognizes, on the one hand, that there are primary and paradigm exercises of power, namely in per se instances of efficient causation, and yet, on the other hand, not all exercises of power bring about their effects in a uniform way. While per se causes are simultaneous with their effects, advising and preparing causes act prior to their effects. While unimpeded per se causes necessitate their effects, other efficient causes do not necessitate their effects. Despite their difference, what all instances of efficient causation have in common is that the cause influences the being of its effect by exercising a power in action. By conceiving of causation as an ontological relationship of dependence, Aquinas is able to find a common feature between pairs of causes and effects that bear different temporal, modal and logical relationships to each other. This gives his view flexibility to account for varied instances of causation which might pose counterexamples to more rigid views that define causation in terms of a single logical relationship.