The Lowenheim-Skolem theorem says (loosely interpreted) that even if you CLAIM to be talking about uncountably infinite things, there’s a perfectly self-consistent interpretation of your talk that refers only to finite things (e.g. your definitions and proofs themselves).
Only in first-order logic. In second-order logic, you can actually talk about the natural numbers as distinguished from any other collection, and the uncountable reals.
Amusingly, if you insist that we are only allowed to talk in first-order logic, it is impossible for you to talk about the property “finite”, since there is no first-order formula which expresses this property. (Follows from the Compactness Theorem for first-order logic—any set of first-order formulae which are true of unboundedly large finite collections also have models of arbitrarily large infinite cardinality.) Without second-order logic there is no way to talk about this property of “finiteness”, or for that matter “countability”, which you seem to think is so important.
You can capture the property “finite” with a first-order sentence over the “standard integers”, I think. This leaves open the mystery of what exactly the “standard integers” are, which looks lightly less mysterious than the mystery of “sets” required for second-order logic.
Proof theory for second-order logic seems to be problematic, and I have a formalist stance towards mathematics in general, which leads me to suspect that the standard definitions of second-order logic are somehow smuggling in uncountable infinities, rather than justifying them.
But I admit second-order logic is not something I’ve studied in depth.
Yeah, second-order logic is basically set theory in disguise. I’m not sure why Eliezer likes it. Example from the Wikipedia page:
There is a finite second-order theory whose only model is the real numbers if the continuum hypothesis holds and which has no model if the continuum hypothesis does not hold. This theory consists of a finite theory characterizing the real numbers as a complete Archimedean ordered field plus an axiom saying that the domain is of the first uncountable cardinality. This example illustrates that the question of whether a sentence in second-order logic is consistent is extremely subtle.
Amusingly, if you insist that we are only allowed to talk in first-order logic, it is impossible for you to talk about the property “finite”, since there is no first-order formula which expresses this property.
An equivalent (and in my opinion less misleading) way of putting this is to say that there’s no first-order formula which expresses the property of being infinite.
Only in first-order logic. In second-order logic, you can actually talk about the natural numbers as distinguished from any other collection, and the uncountable reals.
Amusingly, if you insist that we are only allowed to talk in first-order logic, it is impossible for you to talk about the property “finite”, since there is no first-order formula which expresses this property. (Follows from the Compactness Theorem for first-order logic—any set of first-order formulae which are true of unboundedly large finite collections also have models of arbitrarily large infinite cardinality.) Without second-order logic there is no way to talk about this property of “finiteness”, or for that matter “countability”, which you seem to think is so important.
You can capture the property “finite” with a first-order sentence over the “standard integers”, I think. This leaves open the mystery of what exactly the “standard integers” are, which looks lightly less mysterious than the mystery of “sets” required for second-order logic.
Yes, that’s my understanding as well.
Proof theory for second-order logic seems to be problematic, and I have a formalist stance towards mathematics in general, which leads me to suspect that the standard definitions of second-order logic are somehow smuggling in uncountable infinities, rather than justifying them.
But I admit second-order logic is not something I’ve studied in depth.
Yeah, second-order logic is basically set theory in disguise. I’m not sure why Eliezer likes it. Example from the Wikipedia page:
An equivalent (and in my opinion less misleading) way of putting this is to say that there’s no first-order formula which expresses the property of being infinite.