Source: pp 61-62, Think: A Compelling Introduction to Philosophy (1 ed, 1999) by Simon Blackburn
We can put the issue like this. Imagine God creating the universe. How much does he have to do? One attractive doctrine would be this: he has to create the physical stuff and the laws of physics, and then everything else follows. On this view, by fixing the physical state of the universe at all times, a creating God fixes everything at all times.
[1.] If he had wanted to make a world in which something was different -- say, one in which pinpricks were not painful -- then he would have to have tinkered with the physical facts so that this did not come about.
He would have had to fix up different nerves and pathways in the body and brain.
[2.] There is no independent variation whereby the physical could stay the same, but the mental be different.
This is Leibniz's position, at least as it appears in this passage. (A different interpretation of Leibniz has him thinking that there is independent variation but God has, of course, chosen the best way of associating mental and physical events.)
Locke, on the other hand, thinks that God has two different things to do. First, fix all the physics and laws of physics. But second, decide how to "annex" mental events to physical events, fixing up psycho-physical relations. It is as if the world has two different biographies, one of its physical happenings and one of its mental happenings, and God had to decide how to relate them. On this account, there could be independent variation. God could have kept the physics just the same, but decided not to annex pain to pinpricks.
2 appears irrelevant to 1. Why would 1 need an independent variation of the world? To achieve 1, (omnipotent) God could have incorporated this difference (painless pinpricks) while creating the original world, by excluding pinpricks from pain and leaving the same everything else.
I might have misunderstood 2; I am confused by the concept of 'independent variation'.