So Aristotle's argument for a prime mover is weak because Aristotle's understanding of causation is limited?

Sorry if I misunderstood, but is that the weakness of the prime mover argument?

Also, can you give me an example or explanation of motion as an essence? Would this mean that motion is a substance?

I didn't say that. I said the picture is much larger and more complex than it seems.

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Motion is not a substance. (Indeed, what is a substance? Just an analogical term, reflecting Aristotle's Material cause. But that's a wider issue.)
Examples of motion as essence are plentiful. The essence of a runner is that he or it runs. The essence of a clock is that it measures or reflects the passage of time. And so forth.
Also, the purposes of man-made objects are often motions, note well. Like the said clock. For example, a car is a vehicle made to move people. This is Aristotle's fourth cause - the Final cause.
There is also the quasi-purposive in Nature - for instance the functions of bodily organs in the sustenance of the organism's life.

Dave, I suggest we stop this conversation and you read some of my works on causality. You'll find all answers there, if you are really looking for them. Best regards, Avi.

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