We're concluding our treatment of Dialogues on Metaphysics and Religion (1688) with a full discussion on a different day from parts 1 and 2. In this preview, we focus on dialogue 6 where M. says why a proof of the existence of the external world isn't possible, yet we should believe it anyway on the basis of "revelation," which means both Biblical (the Bible talks about things created) but also the "natural revelation" that sensation itself provides.
So what is this natural revelation? We have to take one more swipe at M's epistemology: Sensations are implanted in us by God according to His nature, which is to say the laws of nature. And we then have to use reason to evaluate these sensations and say which ones actually reveal things in the external world, and which might be illusory, because we as fallen creatures (original sin) can't read God's signals without error and confusion.
But this reasoning about the existence of objects will not be deductive: There's a lot we can conclude from the definition of God, but not that He must have created the world. We instead need faith, which for M. is an essential part of practical reason: Just to know that when we take a step, that the world will still be there to step on takes faith. Likewise, while we could like Descartes doubt the existence of a world, as a practical matter, we need to trust that it really does exist. M. is claiming that this kind of faith and faith in scripture amount to the same thing.
Just to remind you, Wes' acronym GLUSAB stands for general laws of the union of soul and body.
In the full episode, we also discuss dialogues 8 and 9, where M. gives his response to the problem of evil (his theodicy, which was prior to and influential on Leibniz's famous theodicy). In short, God's impressive nature means that He acts simply, which is the same as saying that natural laws are simple, which is the assumption that scientists make. Were God to take our suffering into account in designing His universe, then He'd be constantly having to make exceptions to natural laws, because suffering follows causally from other features of the system. For example, astronomic movements result in weather patterns, and these patterns can ruin your day or even destroy your life.
This would be a fine argument (like Spinoza's) about the impossibility of miracles, but Malebranche is too orthodox in his beliefs to allow that. So it turns out that God does make exceptions to natural law in the case of miracles, but it's not like He just decides on the spur of the moment to make an exception. Rather, the web is just a bit more complex than we thought (or at least looks that way to us mortals) and was preordained from the beginning of time to include those miracles.