Volume 115, Issue 9, September 2018
Peter Koellner
Pages 453-484
On the Question of Whether the Mind Can Be Mechanized, II: Penrose’s New Argument
Gödel argued that his incompleteness theorems imply that either “the mind cannot be mechanized” or “there are absolutely undecidable sentences.” In the precursor to this paper I examined the early arguments for the first disjunct. In the present paper I examine the most sophisticated argument for the first disjunct, namely, Penrose’s new argument. It turns out that Penrose’s argument requires a type-free notion of truth and a type-free notion of absolute provability. I show that there is a natural such system, DTK. I prove a series of results which show that (1) Gödel’s disjunction is provable in the system, (2) Penrose’s argument is invalid in the system, (3) there can be no proof or refutation of either disjunct in the system, (4) the independence results are robust in that they persist when one strengthens the principles governing absolute provability, and (5) there are reasons to believe that the situation will not improve under any plausible alteration of the underlying theory of truth.