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Paper   IPM / M / 16732
School of Mathematics
  Title:   A note on fixed points in quantified logic of proofs and the surprise test paradox (In Persian)
  Author(s):  Meghdad Ghari
  Status:   Published
  Journal: Logical Studies
  Vol.:  12
  Year:  2021
  Pages:   129-153
  Supported by:  IPM
In this note, we study the effect of adding fixed points to justification logics. By making use of the fixed point operators (or diagonal operators) introduced by Smorynski in his Diagonalization Operator Logic, we introduce fixed point extensions of Fitting's quantified logic of proofs QLP. We then formalize the Knower Paradox and various self-reference versions of the Surprise Test Paradox in these fixed point extensions of QLP. By interpreting a surprise statement as a statement for which there is no justification or evidence, we propose a solution to the self-reference version of the Surprise Test paradox. We show that one of the axioms of QLP (the Uniform Barcan Formula) could be the reason for producing contradiction in these paradoxes, and thus by rejecting this axiom we can avoid contradiction in the aforementioned paradoxes. By introducing Mkrtychev models for the fixed point extensions of QLP, we further show that these fixed point extensions (without the Uniform Barcan Formula) are consistent.

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