Logos and episteme
Johnny SAKR
ABSTRACT: This paper advances a conditional rationalist argument against the metaphysical possibility of backward time travel. It proceeds from the assumption, accepted by some but not all modal rationalists, that metaphysical possibility is constrained by intelligibility, where intelligibility requires non-circular explanation and justification. Under this assumption, worlds that violate either the Strong Principle of Sufficient Reason (Strong-PSR) or anti-circular epistemic norms fail to meet the constitutive standards of metaphysical admissibility. The paper formalises this link through a Bridge Principle, showing that any non-trivial backward time travel on a single timeline inevitably generates either causal loops (self-grounding contingent totalities) or epistemic loops (self-justifying knowledge states). Both forms of circularity undermine explanatory and justificatory structure at the global level. While logically consistent and physically modelable, such worlds collapse under the rationalist requirement that intelligibility is non-circular. The conclusion is conditional: if one accepts Strong-PSR or an equivalent acyclicity constraint on grounding and knowledge, then non-trivial backward time travel is metaphysically impossible.
