Free will and the necessary evil

[Belacqua]

So Aristotle thought that everything must have a reason why it's so. 

Which erroneous explanation was based on this assumption alone, and not an incorrect understanding of, say, physics?

Some physical events just happen (Casimir effect); they do not have a reason for happening, rather, they just happen.

I don't think Aristotle addressed the Casimir effect. 

Do you still think that the PSR is why he got rates of falling wrong? 

Anyway, as you know, when Aristotle uses the word "cause" he means all the things that are necessary for a thing to be the case. This is a broader meaning than our normal usage, which he would call "efficient cause." So in Aristotle's terms, the Casimir effect has numerous causes, because a number of things have to be the case for it to happen. For example, time and space have to exist, laws of nature have to exist, various kinds of fields or whatever have to exist. 

I don't know if it's acceptable to say that "it happens because the laws of nature are the way they are" counts as a "sufficient reason" or not, PSR-wise.