Philosophy Colloquium
Jenann Ismael
University of Arizona
The Paradox of Predictability Revisited

In 1814 Pierre LaPlace drew an epistemic consequence of the determinism of the Newtonian equations of motion that is quoted in almost every philosophical discussion of determinism, writing:

“An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes. “

But what if there were a device that was specifically designed to take a prediction of its behavior and do the opposite? Would a LaPlacean intelligence with knowledge of the laws and the initial state of the universe be able to predict the behavior of a device like this? You might be tempted to think a device like this is impossible in a deterministic world. In fact, they are quite trivial to construct. The puzzle presented by counterpredictive devices was introduced into the philosophical literature by a 1965 paper by Michael Scrivens as the “Paradox of Predictability”. A number of people wrote papers about the Paradox in the late 1960s and early 70s, but the discussion ended somewhat inconclusively, because there were features of the original presentation that unnecessarily diverted the discussion. I want to revisit the ‘Paradox’, present it a little more cleanly, and try to understand it as a purely physical phenomenon. It holds some important lessons about determinism and the nature of physical laws.

Wednesday, November 11, 2015

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