This paper extends SΔϕ-25 by providing a minimal operational criterion for event fixation. Whereas SΔϕ-25 introduced pre-detection as the condition in which difference is already operative before it becomes detectable or eventfully registered, the present paper asks when such difference becomes irreversibly fixed. The central claim is that irreversibility should not be defined as absolute impossibility of reversal, but as the condition in which restoration cost exceeds the threshold of practical return. Under this formulation, reversibility is not a matter of formal logical possibility alone. A transition is reversible only when restoration remains available at sufficiently low cost. Conversely, a transition becomes irreversible when restoration cost rises beyond the threshold at which return is no longer practically selectable. The paper introduces a minimal distinction between formal reversibility and practical irreversibility, defines event fixation in terms of restoration cost structure, and proposes that digital and virtual environments do not necessarily weaken irreversibility. On the contrary, by lowering propagation cost while increasing restoration burden, such environments may intensify irreversible fixation. Within the SΔϕ series, this paper clarifies the missing operational criterion between latent difference and fixed event, and redefines irreversibility as restoration cost explosion rather than metaphysical finality. Series position: SΔϕ-25 — Pre-detection as Latent Difference: Minimal Event Conditions before Irreversible Registration (v1.0) SΔϕ-26 — Irreversibility as Restoration Cost Explosion: Minimal Fixation Conditions after Pre-detection (v1.0)
Sofience (Sat,) studied this question.