Identity, Causality, and Caterpillars

Identity, Causality, and Caterpillars

Suppose you don’t know the true relationship between caterpillars and butterflies: you don’t know that the former turn into the latter by metamorphosis. You notice the two are often found in the same vicinity, but you don’t know that they are really the same organism at different stages of its life. You assume they belong to different species. You decide to investigate these creatures: you place five butterflies and five caterpillars in a sealed container to see what happens. Lo and behold, when you return some time later you find there are now ten butterflies and no caterpillars—but you never witnessed the transition through the chrysalis stage. You form the hypothesis that caterpillars cause butterflies—without being butterflies. For some reason the one causes the other, as if a flea could cause an elephant; it seems miraculous but you feel driven to accept it. Things cause other very different things all the time. Now suppose you conduct more diligent observations and come to the conclusion that actually they were the same organism all along, but with a different appearance at different times; you have discovered metamorphosis. Question: would you conclude that you were wrong earlier to suppose that caterpillars cause butterflies? I think not: you would hold that the causal claim is true despite the identity—that the two propositions are logically (metaphysically) compatible. And you would be right to do so, in my opinion. Isn’t it just straightforwardly true that caterpillars are the cause of butterflies (not flowers or bees or hummingbirds)?

Now suppose you are an alien visiting Earth and you observe babies and adults, but you don’t know that babies grow into adults while retaining their identity. You think these are creatures of different species that are often found in the same vicinity. After some experiments (see above), you conclude that babies cause adults but are not the same as them (they look so different). Then later, after more careful observation, you reach the startling conclusion that babies and adults are the same creatures at different stages of a single life. Do you revise your earlier causal judgment? No, you conclude that there is a causal relationship between them and that they are identical. Identity does not preclude causality. You would be right to do so, in my opinion.

Now imagine you are an astronomer investigating the planets, about whose nature you know very little; you vaguely suppose they are big fireballs in the sky. One you call “Hesperus” and another you call “Phosphorous”; you get it into your head that Hesperus causes Phosphorus—one fire in the sky ignites another fire in the sky (you don’t know how). Then you find out these two objects are actually the same hunk of rock hurtling through space. Well, I never, Hesperus is Phosphorus! Do you abandon your earlier causal judgment? You see no reason to; it has merely turned out that Hesperus and Phosphorus stand in both relations—identity and causality. The thing you see in the evening is the cause of the thing you see in the morning (what it does during the day you have no idea). Nothing else does—not the Moon or the Sun or Mars.

You have seen Superman and Clark Kent around and you begin to suspect a connection. Is one of them the cause of the other? It seems unlikely that Clark is the cause of Superman, given the former’s rather unimpressive persona, so you conjecture that Superman is the cause of Clark. No way they can be identical, given their very different abilities and sartorial style. Then one day you see Superman actually changing into his Clark Kent clothes and you decide, rightly, that the two are identical. Not only does Superman cause Clark Kent but the latter is the former, surprisingly enough. Again, you feel no inconsistency—correctly, in my view. You also note that causal contexts are not always referentially transparent (you can’t say “Clark Kent is the cause of Superman”, even though the two names have the same reference).

You are a budding physicist and chemist interested in heat, light, and water. You have been reading about molecules, photons, and gases, and you form some hypotheses: heat is caused by molecular motion, light is caused by streams of photons, and water is caused by molecules of oxygen and hydrogen. These hypotheses have notable explanatory virtues; you are hoping for a Nobel Prize. On reflection and further experimentation, you arrive at the view that heat is molecular motion, light is a stream of photons, and water is H2O. Does that mean you were wrong in your causal claims? I don’t think so: identity and causality are compatible. Water is causally explained by H2O and water is identical to H2O.

Finally, you are a neuroscientist interested in the brain-mind connection. You are convinced that the brain causes the mind and you assume they are distinct things (you might even be a Cartesian dualist). Later, you become persuaded that the mind-brain identity theory is true—should you give up your causal claims? Again, why should you—the two things are compatible. Believing the identity theory does not rule out believing the causal theory, both of which seem eminently defensible. We can have theoretical identification and causal relations. Pain can be an effect of C-fiber firing as well as being identical to it.

It is true that these doctrines are contrary to the way philosophers have talked of identity and causality for many years, but these ways of talking seem not to fit the intuitive facts. Causal relations don’t always require numerical distinctness.[1] We have to get used to new ways of talking.

[1] See my “Causality and Identity”. The present paper is intended to provide some intuitive data in support of that earlier paper. It doesn’t substitute for it.