Flies and Fine-tuning


Charles: Consider our universe. It seems ‘fine tuned’ for life. Many of the fundamental dials of our universe need to be set ‘just so’ within an exceptionally small degree of tolerance for our universe to remain ‘life friendly’. Assuming these values can range fairly freely, that we arrived at this combination is ludicrously unlikely ‘just by chance’. If God exists though, we’d expect a universe he creates to be life friendly, because life is part of his design plan. So the fine-tuning of our universe confirms Theism over Atheism.

Debbie: Several problems. Good metaphysical sceptics like me won’t be happy to say how, or in what sense, the relevant values can ‘range freely’ (maybe they’re necessary). Ignore that though – I suspect these worries can be answered. I’m more worried about a multiverse. Take some sensible principle like “all possible worlds exist” or “every possible combination of these values exist”. If that is true, then we shouldn’t be surprised at all to see that we exist, even in a universe finely tuned (or even uniquely tuned) for life.

Charles: That seems ad hoc to me. What reason is there to suppose a multiverse, but for Atheists to explain away fine tuning data?

Debbie: Ad hoc isn’t the be-all and end-all of explanatory virtue. After all, we all should develop and modify hypotheses in light of new data. I believe some think many worlds because of certain mysteries of quantum mechanics, so for these people it isn’t an ad hoc addition to protect their atheism. Regardless, I see no reason to deny a multiverse, even if I don’t have much reason to affirm one. I should leave it as a live option. Yet, if this is a live option, then the confirmation of Theism over Atheism seems to be lacking: fine tuning becomes unsurprising because it is unsurprising on a multiverse, and that itself is unsurprising given Atheism.

Charles: Maybe so. But even if we get to ‘unsurprising’, that isn’t as good as exactly what we’d expect. Given Theism, fine tuning is exactly what we’d expect, whilst on Atheism, it is (given the live option of a multiverse) merely unsurprising. Suppose I know of two urns, one filled with white balls, and one filled with a fifty-fifty mix of white and red. I draw out a white ball. That’s unsurprising on the half-and-half urn, but exactly what you’d expect on the all-white urn. So that acts to confirm the latter.

Debbie: Sure, but I have more than a few worries about whether we should really expect life given Theism. I’m not convinced perfect beings would be motivated to create anything at all. These worries are insiginificant if you can show the fine tuning data is ludicrously unlikely on Atheism, but spring to the forefront if it’s only unsurprising.

Leave that aside, though. I think there’s a bigger worry – I don’t the argument works even if we restrict ourselves to a single universe.

Modal scope

Debbie: I don’t think we can see enough of the relevant possibilities to pass any judgement on whether life-friendliness is surprising given Atheism. Among the space of all possible worlds, we can surely inspect very few of them: it seems likely that the number of worlds we can examine is a tiny subset of these. Why not worlds where there are no physical parameters, and just filled with consciences? Or any number of things I can dream up (and, further, knowing that will be still other possibilities I can’t). Yet inferring from our tiny subset of possible worlds of which barely any are life-friendly to infer that it’s life-unfriendly across the board is an outrageous extrapolation.

Charles: Sure, but we’re only talking about epistemic possibilities: so we can begin to filter out worlds we can’t imagine, or worlds we might think to be unlikely. Maybe physicalism is true and so worlds of disembodied consciences simply can’t happen. Or maybe, in fact, there is some necessary set of physical parameters.

Debbie: There might be. But why believe that? You presumably don’t want your argument to hinge on any bold or controversial philosophical theses. If you want to say there is some necessary set of physical parameters (so excluding out all of these hypothetical worlds I can dream up as not real possibilities) I can counter-offer that we shouldn’t stop there: let’s say the particular values of said parameters are necessary and so these hypotheticals where the universe has slightly different values and stops being life permitting are likewise non-possibilities.

Charles: I see. Let’s diagram it out:


The picture represents a field of all the ways, for all we know, the universe could have turned out. Red is life-permitting, and the dot is our universe. The white circle around it the world we can examine that are life-unfriendly, and surround our uinverse. The grey is the limit of our epistemic spotlight, those epistemic possibilities we believe are there but cannot evaluate their life-permitting. For the fine-tuning argument to work, we need to believe that white is much, much more common that red.

So your objection, I take it, is that our spotlight is far too small to see that. There’s a simply vast expanse, and we can’t legitimately infer from the rarity of life in our epistemic locality to its global rarity. What I would want to do is cut out these epistemic imaginings by saying we can conclude on reflection large chunks of this environment you suggest can’t exist – and, ultimately, that the space we can survey takes up the bulk of the worlds that we should accept as live possibilities.

Debbie: That sounds right. I think the only way you’ll be able to cut that down so radically is say something like: all possible worlds share the same core set of physical parameters as our own (even that may not be enough – can we evaluate worlds with all of our constants changed by hundreds of orders of magnitude?) Yet I see no reason to accept that sort of claim – certainly no more so than simply stating that the only possibilities is our own universe. That the constants simply had to take these values (after all, they are nomologically necessary – why not metaphysical too?) If so, we don’t even need a multiverse to exhaust the probability space – we can simply conclude we’re not in a position to evaluate it.

Targets and prediction

Charles: Hmm. In the same way you granted me a single universe to make this objection, I think I can grant this concern and still run the fine-tuning argument. Suppose the epistemic scales are lifted from our eyes, and we see, in fact, that outside our locality, all worlds are life-friendly:


Our world is a bit like a bullseye. Suppose know two options of picking randomly or aiming for a target. Our ‘aiming’ hypothesis seems to be confirmed if we hit our solitary universe in the middle of all this barrenness. This is the famous flies objection.

Debbie: That’s interesting, you’ve changed the game markedly. In this case, we aren’t confirming the aiming hypothesis over the random selection because our result is life-friendly: given your example, the vast chunk of possibilities are so it would be no surprise to find one ‘at random’. What’s surprising here is there seems something special about this lone universe – this lone fly: that it’s on it’s own in a field. So it’s no longer the fact that our universe is life friendly that’s causing the surprise, but rather that it’s tune d to be in a locality of non-life-permitting universes.

Charles: I suppose so. Is that problematic?

Debbie: I think so, yes. The problem is I don’t see any reason to accept this sort of ‘targeting hypothesis’. I can see (with reservations) why God would want to produce a life-friendly world. I can’t see at all why we’d expect God to want a ‘bullseye’ world. Without this, there’s no confirmation.

Charles: What about giving a sign he exists? That the appearance of fine-tuning is something important about the world?

Debbie: That seems loaded with problems. Theists can’t agree to what extent God should give signs and to what extent he should conceal himself. Also, there’s something dialectically weird here: we’re arguing precisely about whether apparent fine-tuning points to God. If I’m right and this appearance shouldn’t point us to God, God including it to point us to him seems bizarre: it would only be a sign to the insufficiently reflective.

I think there’s a general problem here too. Pick any ‘Godly motivation’ extension for why God would want our ‘bullseye’ universe. Pulling down our epistemic veils, we have no reason to think there wouldn’t be an even more ‘bullseye’ universe out there in the mist. In other words, whatever properties you point of our modal locality that God would want, it seems probable that there would be a locality even more desirable.

Charles: I accept your first point, but not your second. If I can come up with good reasons as to why God would want a ‘bullseye universe’, then I don’t think I need to be worried by the possibility (or probability) there would be a universe that makes an even ‘better’ bullseye. Epistemic space is very big – there’s plausibly an infinite succession of such universes. God is not at fault for actualizing a non-maximal one.

Debbie: That’s reasonable, although I worry about whether the ‘infinite succession of worlds’ applies to infinite succession of regions of worlds. Regardless, you’re still stuck. It seems hard to make a good auxiliary prediction from God – at least one as strong as you’d have from ‘just’ permitting life.

Charles: Yes. I think I need to close off the outs you can take. I need to knock out the multiverse hypothesis, or show somehow that fine-tuning concerns apply.

Debbie: That might be fairly hard. How about a ‘simple’ principle like possible world actualism? The other worry is you need to prune our epistemic possibilities. You need to avoid getting sucked into the modal explosion that leaves our epistemic spotlight as unable to grant us access. That seems hard.

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