Interaction and the spear thrower

Lucretius argued for the infinity of the universe by imagining a spear-thrower at its edge. We can’t imagine any finite space that isn’t contained by a greater, and so there must be spaces within spaces forever.

The interaction problem extends this line of thinking to effects in space. Say you come to the edge of a physical effect. The energy by which it’s moving has to come from somewhere. You could even poke at it with your spear, right?

We can no more imagine a non-interacting effect than a non-contained space, but contemporary persons are more comfortable with a finite universe than a spiritual cause of physical effects. Presumably we’d claim this is because of the demands of physical theory – GTR demands a (sort of) finite universe but the conservation of energy or Newton’s Third Law demands interaction of physical causes and effects. I think the argument fails for a few reasons:

1.) The ontology of energy and force is not defined enough even to justify the claim that interaction is more probably true. Asking what energy is very quickly collapses into either contradiction or pure mathematical convenience.

2.) The ontology of what interacts energetically is also poorly defined. STA would treat “energy” as an accident of moved movers, and to do so changes nothing in the equations.

3.) Plantinga notes that conservation laws apply only to closed systems, and he denies that the universe is a closed system.

At any rate, interaction problems are all composition fallacies that try to bootstrap from facts about interactive systems to causal orders. But interactive systems are only accidentally causal since, as Newton points out (and this is the real ontology of the Third Law) the difference between cause and effect in interactive systems as such is only logical and does not involve an essential order.


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