nshepperd comments on Conservation of Expected Evidence - Less Wrong

68 Post author: Eliezer_Yudkowsky 13 August 2007 03:55PM

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Comment author: nshepperd 03 October 2016 04:50:52AM 0 points [-]

"For a true Bayesian, it is impossible to seek evidence that confirms a theory"

The important part of the sentence here is seek. The isn't about falsificationism, but the fact that no experiment you can do can confirm a theory without having some chance of falsifying it too. So any observation can only provide evidence for a hypothesis if a different outcome could have provided the opposite evidence.

For instance, suppose that you flip a coin. You can seek to test the theory that the result was HEADS, by simply looking at the coin with your eyes. There's a 50% chance that the outcome of this test would be "you see the HEADS side", confirming your theory (p(HEADS | you see HEADS) ~ 1). But this only works because there's also a 50% chance that the outcome of the test would have shown the result to be TAILS, falsifying your theory (P(HEADS | you see TAILS) ~ 0). And in fact there's no way to measure the coin so that one outcome would be evidence in favour of HEADS (P(HEADS | measurement) > 0.5), without the opposite result being evidence against HEADS (P(HEADS | ¬measurement) < 0.5).