Hume’s arguments
In §VII, Hume sought to “fix, if possible, the precise meaning” of such terms as “force”, “power”, “energy”, and “necessary connection” (between cause and effect) (§VII, par. 3). In our terms, this is the project of distinguishing between cause and coincidence: a cause makes its effect happen whereas two things that are only coincidentally related have no bearing on one another.
Hume’s conclusion was that the most that can be done is to define causes in terms of regularities (§VII, par. 29). That is obviously not what most people think is involved in a cause. We think that causes do something to bring about effects. But it’s the most Hume thinks we can make sense of.
His argument began with his principle that all ideas are copied from impressions. The implicit suggestion is that we can’t mean anything by one of those words if we do not have an idea corresponding to the word.
Then he surveyed two possible sources of the impression of force, power, energy, or necessary connection: our sensory impressions of external objects and our impressions of reflection of the operations of our own minds.
The first is dismissed with a familiar argument: for every cause-effect pair we see, we can imagine that the effect does not happen. Since a true cause would make the effect necessary, Hume concludes that we do not have an impression of this necessitating force, power, energy, etc. from the senses.
The other option is that we find the impression by considering how our minds work. There are two sub-options. One concerns the mind’s ability to move the body and the other concerns the mind’s ability to call up particular ideas, as when we decide to imagine a particular thing or think about a particular problem.
No one will be surprised to hear that Hume didn’t think we find the relevant impression here either.
Occasionalism
One thing about all of this is puzzling. Why did Hume spend the bulk of his time on the second option?
I think the answer is that he was taking aim at Malebranche’s occasionalism, the doctrine that the events you and I can observe, so-called ‘natural causes’, are simply occasions for God to bring about effects.
Malebranche argued that we can only understand the relationship between cause and effect if it is the product of an omnipotent power. Causes make effects necessary. That means no other result is possible. But what could rule out all possibilities other than the effect that actually happens? Only an omnipotent being.
So, according to Malebranche, causes and effects depend on God’s will, or, God’s mind.
Hume concentrated on the mind in order to make a simple point. We have no idea how God could bring about effects. Calling him “omnipotent” doesn’t help. That just means he has infinite power. But what is power in the first place, whether finite or infinite? That’s Hume’s question.
Nathana’s question about Malebranche
Nathana asked why Malebranche was willing to pay the price of making God responsible for human sins by adopting the doctrine of occasionalism. I’m not sure if there’s an answer to that exact question because I doubt that Malebranche saw it that way. I think he thought occasionalism was true and would not have thought of abandoning it because it raised theological problems elsewhere. In any event, he had answers for the other theological problems concerning free will and God’s responsibility for the apparent evils of the world.
An accessible summary of these views can be found in the Routledge Encyclopedia of Philosophy entry on Malebranche. You can find this on the fourth floor reading room in Regenstein or online (with a University of Chicago internet connection).