komponisto comments on How to Convince Me That 2 + 2 = 3 - Less Wrong

52 Post author: Eliezer_Yudkowsky 27 September 2007 11:00PM

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Comment author: Eliezer_Yudkowsky 14 September 2011 06:26:53AM *  10 points [-]

I don't think people really understood what I was talking about in that thread. I would have to write a sequence about

  • the difference between first-order and second-order logic
  • why the Lowenheim-Skolem theorems show that you can talk about integers or reals in higher-order logic but not first-order logic
  • why third-order logic isn't qualitatively different from second-order logic in the same way that second-order logic is qualitatively above first-order logic
  • the generalization of Solomonoff induction to anthropic reasoning about agents resembling yourself who appear embedded in models of second-order theories, with more compact axiom sets being more probable a priori
  • how that addresses some points Wei Dai has made about hypercomputation not being conceivable to agents using Solomonoff induction on computable Cartesian environments, as well as formalizing some of the questions we argue about in anthropic theory
  • why seeing apparently infinite time and apparently continuous space suggests, to an agent using second-order anthropic induction, that we might be living within a model of axioms that imply infinity and continuity
  • why believing that things like a first uncountable ordinal can contain reality-fluid in the same way as the wavefunction, or even be uniquely specified by second-order axioms that pin down a single model up to isomorphism the way that second-order axioms can pin down integerness and realness, is something we have rather less evidence for, on the surface of things, than we have evidence favoring the physical existability of models of infinity and continuity, or the mathematical sensibility of talking about the integers or real numbers.
Comment author: komponisto 14 September 2011 07:37:31AM 6 points [-]

Everything sounded perfectly good until the last bullet:

why believing that things like a first uncountable ordinal can contain reality-fluid in the same way as the wavefunction

ERROR: CATEGORY. "Wavefunction" is not a mathematical term, it is a physical term. It's a name you give to a mathematical object when it is being used to model the physical world in a particular way, in the specific context of that modeling-task. The actual mathematical object being used as the wavefunction has a mathematical existence totally apart from its physical application, and that mathematical existence is of the exact same nature as that of the first uncountable ordinal; the (mathematical) wavefunction does not gain any "ontological bonus points" for its role in physics.

or even be uniquely specified by second-order axioms that pin down a single model up to isomorphism the way that second-order axioms can pin down integerness and realness

Pinning down a single model up to isomorphism might be a nice property for a set of axioms to have, but it is not "reality-conferring": there are two groups of order 4 up to isomorphism, while there is only one of order 3; yet that does not make "group of order 3" a "more real" mathematical object than "group of order 4".