Each of Aristotle’s four “causes” or kinds of reasons why a thing is the way it is picks out a distinct kind of conceptual content. Actually, none of them — including the efficient cause — should be thought of in terms of anything like a mechanical impulse or force or the exertion of a force. An efficient cause is also not primarily a thing that exerts a force. Rather, an Aristotelian “efficient” cause exercises what in modern terms most closely resembles a sort of structural causality, associated with the form and materiality of the means by which a thing is realized as the sort of thing it is. It acts in an instrumental way that is more “logical” than physical.
In an example of the production of a statue, the efficient cause is not the sculptor, or the sculptor’s will, or the blows of the sculptor’s hammer and chisel. It is the art (objectively characterizable technique) by which the statue is produced. Many people have certainly made contrary assertions about this, but there is, e.g., a good discussion in the Stanford Encyclopedia of Philosophy that supports the above interpretation. (In this simple example, the end is the finished work of the statue. The form and matter are the form and matter of the finished work.)
While the efficient cause is perhaps a little closer to a “cause” in the usual modern sense, it is still far from the same, even though it has a much closer connection to the less common notion of structural causality. Aristotle himself put either form or end first, but influential late scholastics such as Suarez elevated the efficient cause above the other three, perhaps on the ground that God was considered to be pre-eminently an efficient cause (whereas for Aristotle, the “First” cause is primarily an end). I seem to recall some reference to late scholastics treating creation ex nihilo as an example of efficient causality. In any event, Suarez is regarded as treating all four Aristotelian causes on the model of the efficient cause. This helped pave the way for early modern mechanism’s reduction of all causality to a single, univocal form.
Aristotle’s semantically oriented science aims not so much at prediction of what we would call physical events as at a retrospective understanding of why things have turned out the way they have, in a humanly relevant, pragmatic way. Aristotelian “causes” are pluralistic and nonunivocal. They are just reasons why something came out the way it did.
The Postmodern Peripatetic 