It is one of the tasks of the philosophy of mathematics to explain in a Kantian way, how and to what extent, mathematics as we know it, is possible. It should be clear, however, that, as things stand at present, a philosophical theory of mathematics can in this respect, it seems, be little more than a declaration of faith or the lack of it.

We present a decision procedure for the ∀∃ theory of the lattice of Σ1 sentences of Peano Arithmetic.

It is commonly agreed that the well-known Lucas-Penrose arguments and even Penrose's 'new argument' in [Penrose, R. (1994): Shadows of the Mind, Oxford University Press] are inconclusive. It is, perhaps, less clear exactly why at least the latter is inconclusive. This note continues the discussion in [Lindström, P. (2001): Penrose's new argument, J. Philos. Logic 30, 241-250; Shapiro, S.(2003): Mechanism, truth, and Penrose's new argument, J. Philos. Logic 32, 19-42] and elsewhere of this question.

It has been argued, by Penrose and others, that Gödel's proof of his first incompleteness theorem shows that human mathematics cannot be captured by a formal system F: the Gödel sentence G(F) of F can be proved by a (human) mathematician but is not provable in F. To this argment it has been objected that the mathematician can prove G(F) only if (s)he can prove that F is consistent, which is unlikely if F is complicated. Penrose has invented a new (...) argument intended to avoid this objection. In the paper I try to show that Penrose's new argument is inconclusive. (shrink)

We present a quite simple proof of the fixed point theorem for GL. We also use this proof to show that Sambin's algorithm yields a fixed point.

We present a decision procedure for the ∀∃ theory of the lattice of Σ1 sentences of Peano Arithmetic.

Improving on a result of Arana, we construct an effective family (φ r | r ∈ ℚ ⋂ [0, 1]) of Σ n -conservative Π n sentences, increasing in strength as r decreases, with the property that ¬φ p is Π n -conservative over PA + φ q whenever p < q. We also construct a family of Σ n sentences with properties as above except that the roles of Σ n and Π n are reversed. The latter result allows (...) to re-obtain an unpublished result of Solovay, the presence of a subset order-isomorphic to the reals in every non-trivial end-segment of every branch of the E-tree, and to generalize it to analogues of the E-tree at higher levels of the arithmetical hierarchy. (shrink)