OK. I 'll give an example of
per se causal series from the link I provided if you haven't already read it.
Let's say we see an illumined room in the evening. The property in question is illumination. We see that the room is illumined by moon. But moon is not the ultimate cause. We know the moon does not possess the power of illumination inherently. It can just reflect light not produce it. The moon acts as an intermediary (instrumental) cause in this causal series.
That is what makes a causal series (x) -> (y) -> (z) per se: (x) must borrow a power from (y) to produce (z). This is clearly the case with the moon deriving light from the sun to illumine the bedroom. (Pat Flynn)
No, the moon simply reflects light. It is the light, not the sun or the moon, that illuminates the room. The light comes from the sun, reflects off of the moon, and then travels to Earth to illuminate the moon.
If the sun disappeared during the time in which the light travels, the light would *still* illuminate the room even though the sun no longer exists.
The causal series is (sun)-->(moon)--->(room), not, as described, (moon)-->(sun)-->(room).
The sun illuminates the moon, which reflects the light, which then illuminates the room. The moon isn't 'borrowing a power'.
Now let's compare this with
per accidens causal series.
Whereas in a causal series (x) -> (y) -> (z) which is per accidens, (x) does not need to borrow a power from (y) to produce (z). Think Harry begetting Bob and then Bob begetting John. Once Bob is begotten, Harry becomes irrelevant and can drop out without interruption to the causal series, because Bob has the power inherently to beget John. Bob doesn’t need to borrow the power of begetting from his father Harry in the process of begetting (that would be weird); he (Bob) has it in virtue of being a biologically well-functioning male. The point? Because members in a per accidens causal series have the inherent power to produce the effect under consideration, Aquinas grants that we need not trace to anything more primary, hence he maintains that such a causal series could regress infinitely.
But not so with a per se ordered causal series /... / (same)
So Aquinas, as usual, has a messed up analysis. Both are simply causal series.
In both series, a property goes from one thing to another to the final. In the sun, moon, room system, the causal sequence is (sun)->(moon)->(room) and in the descent sequence it it (Harry)->(Bob)->(John).
Just as a terminological issue: it seems to me that the moon's role is the 'accident' and Bob's role is the one inherent in the cause. But of course, both have internal properties that are relevant (the moon's ability to reflect light, for example).