Why would God need an independent variation of the world to allow for painless pinpricks?

Question

_{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.

3. 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.

4. I might have misunderstood 2; I am confused by the concept of 'independent variation'.

Answer 1

The question here is whether or not the mental world can be changed without somehow changing the physical world. What Blackburn means when he speaks of an "independent variation:" there is a change in the mental world without a corresponding change in the physical world.

Consider, for a moment, what it means that pin-pricks behave the same physically: it would mean creating an instance where skin could be broken, blood could be drawn, and yet no pain is registered at all. For this to be possible, we would have to have nerves in the body that look and act precisely the same as our nerve do except they do not register a pin when it breaks the skin and draws blood. In other words, one would have to believe that there was some bizarre miraculous intervention on every occasion of a pin-prick to cause this behaviour.

This is precisely what Leibniz would want to deny. For Leibniz there is a rational order connecting the physical with the mental. To speak of pin-pricks without a corresponding link is to posit a physical world and a mental world with a strong distinction between them: there is no direct tie between what happens in the physical world and what happens in the mental world. On such a view, any connection between the physical and the mental would require a special dispensation from God, a miracle.

So how does this play out with an omnipotent God? Leibniz would not deny that God is capable of bringing about such a state of affairs, but to do so God would need to act irrationally, or, as Leibniz sometimes puts it, less perfectly than God is able to. In creating such a world God would have created a world in which constant miraculous intervention was necessary, which rather cheapens the sense of miracles when they do occur, and, moreover, makes our knowledge of the workings of the world (science) impossible, since the world would work in ways that were entirely arbitrary.