Modal Skepticism, Iron and Water

I am working my way through Professor Josh Rasmussen’s and Professor Felipe Leon’s excellent book, “Is God the Best Explanation of Things. A Dialogue.” and was struck by an illustration Professor Leon provided regarding our modal intuition.

Professor Leon attempts to justify a moderate modal skepticism, giving the following illustration to illuminate this point. Dr. Leon begins with imagining an iron bar floating on water. He explains that he can imagine or conceive of such a thing as being possible, but under further investigation he realizes that such an thing is not possible. He explains that in order for such a thing to occur, iron would need to cease being iron, or water water, and a different thing altogether such that their natural properties could bring about the state of affairs where an iron bar floats on water.

I was, admittedly, confused with this example. Within moments my mind wandered, as it tends to do, towards solving the puzzle of how an iron bar might float on water, thereby undermining Dr. Leon’s illustration. Several examples came to me within just minutes, as I am sure they could come to you.

  • A current of water exactly the size of the iron bar could be flowing upwards beneath it with just enough strength to keep the bar afloat and in-place.
  • A force could be pushing all of the water towards the location directly under the iron bar such that the water exerts just enough pressure to keep the bar afloat.
  • A magnetic force above the iron bar could be strong enough to attract it upwards until the downward pressure of the bar was low enough that it would float on water but not be pulled away from the water.
  • There could be a hydrophobic coating or preparation of the iron bar such that it does not sink.
  • The iron bar could be spread so thin/flat as to not break the surface tension.
Pure Iron Bar

Now, you may be thinking at this point that what I have described above is cheating. I have added conditions which make it possible, conditions that Professor Leon would intended to exclude. Well, what are all these conditions that Dr. Leon must set for his argument to run through? What Professor Leon seems to imagine is pure water (H20) and a pure iron (Fe) bar of a certain size, let’s say 1 foot long looking something like this image to the left of this paragraph. The water must not be under any type of sufficient pressure to change its density (preferably at our current atmospheric pressure), there must be no magnetic fields exerting sufficient influence over the iron to change its behavior, no forces such as wind or currents, not modifications like hydrophobic preparations of iron, etc.

Well, is this a fair critique? Perhaps it is, but I believe it raises an alternate criticism of Dr. Leon’s illustration. For Dr. Felipe’s “moderate modal skepticism” to succeed with this illustration, he must describe a very specific scenario down to the molecular structure of the objects involved. This is quite different from imagining a world of Chalmer’s Zombies or Davidson Swampmen, where we suppose many things can change in order to bring about these creatures. On the contrary, Dr. Leon’s argument succeeds only if a broad set of factors are constrained such that the relationship between the items being conceived is physically impossible. Under these terms, it is no more surprising that Dr. Leon’s illustration is impossible than had he imagined that same bar of iron passing through a wall of steel. The physical properties that prevent a bar of iron passing through steel are the same as those that prevent an iron bar from floating on water under the conditions required of Dr. Leon’s illustration. Dr. Leon, inadvertently or not, has slipped into the thought experiment a set of criteria that makes it impossible after it first appeared possible. This isn’t an argument, it is a parlor trick (intentional or not).

Concluding Thoughts

What has come to be known as “moderate modal skepticism” may in fact simply be a lack of imagination or the intentional production of thought experiments which lead to the author’s preferred conclusion. Even Van Inwagen’s canonical example of whether it is possible that iron could be transparent turns out to be probably yes. Perhaps our modal intuitions are more accurate than contemporary epistemology would care to admit. Or was that sentence unknowable as well?

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