Right, cause vs the instrument. Who is responsible for the cause, and who is the instrument.
But, you see, in scholastic terms, an instrument IS a cause, referred to as an instrumental cause. We're just talking about two different causes.
It's like if I pick up a pen and write a letter. I'm the formal cause of the contents of the letter, but the pen is the instrumental cause. There could be no letter written if I did not have the pen.