Modern thinking about causality remains haunted by the figure of “driving” causes on the model of billiard ball impacts, even though the use of sophisticated statistical methods in contemporary science already suggests the obsolescence of that model. I have characterized ordinary Aristotelian actuality as what is effectively operative in a process. This more general “pragmatist” notion of determination pays much more attention, e.g., to the role of the forms of things in shaping processes, but it also subsumes everything that was discussed on the narrower “driving cause” model.
Within the later tradition, the grounds for the early modern reduction of causality to an impact-modeled “driving” sense were laid by scholastic redefinition of Aristotelian efficient causality in terms of activity, and its simultaneous elevation above final and formal causes. I think that for Aristotle, the defining characteristic of an efficient cause is that it is a means by which an end is actualized.
The complete means by which an end is actualized undoubtedly involve activity, but in the example of building a house, for Aristotle it is the art of building that is the pre-eminent efficient cause of the actualized house — the pre-eminent means by which the building of the house is accomplished, even though there are many other contributing means and other causes. But the art of building is not itself an activity. This tells us that activity is not the defining characteristic of an efficient cause. It also brings the notion of efficient cause closer to those of final and formal cause. Taking into account the close relationship of Aristotelian form and matter, it turns out that all four of Aristotle’s causes are integrally related to one another.
I think that for Aristotle, activity is more a practical notion associated with broadly ethical doing, than a basis for theoretical explanation of events and properties. In Aristotle, activity is something inquired about. It does not play the role of an unexplained explainer. (See also Agency; Efficient Cause; Efficient Cause, Again.)