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From Arrow to Historicity

The arrow of time establishes that irreversibility emerges in statistical systems through entropy. This page explores how the literature connects that thermodynamic arrow to the broader phenomenon of historicity — the capacity of systems to retain traces, accumulate structure, and become historical.

From irreversibility to traces

Hans Reichenbach provided the physical mechanism in The Direction of Time (1956). His concept of branch systems explains how traces form: a subsystem interacts with a larger system, acquires a low-entropy imprint through that interaction, and then evolves in quasi-isolation toward equilibrium. A footprint in sand, a fossil in rock, a crater on the moon — each is a branch system whose current ordered state is only explicable by reference to a past interaction.

The asymmetry of traces follows from the asymmetry of entropy. We find traces of the past, not of the future, because branch systems inherit their low-entropy initial states from interactions that occurred when the larger system was at lower entropy. As David Albert argues in Time and Chance (2000), the cosmological low-entropy initial condition — the Past Hypothesis — is what grounds the reliability of all records. A record is informative about the past only because the recording device was in an initialised ready state before the interaction.

Traces are where thermodynamics and information meet. As Landauer established, creating and maintaining a record is a thermodynamic process. Carlo Rovelli formalised this in “Memory and Entropy” (Entropy 24(8):1022, 2022): trace formation requires systems separation, temperature differences, and long thermalisation times — conditions that are “largely satisfied in our universe.” His conclusion: all macroscopic information originates from past low entropy.

From traces to retention

A trace is passive — it is left behind. Retention is active — it is carried forward. The distinction marks a threshold in the literature.

Schrödinger identified the threshold in What is Life? (1944): a living organism avoids equilibrium by feeding on negative entropy. It does not merely carry traces — it actively maintains its own order, importing low entropy from its environment and exporting high entropy in return. The organism is not a branch system drifting toward equilibrium; it is a system that sustains its own distance from equilibrium.

Prigogine generalised the point. Dissipative structures maintain themselves far from equilibrium through continuous entropy exchange. In From Being to Becoming (1980), he frames this as the difference between a physics of being — deterministic, time-symmetric, without history — and a physics of becoming, where irreversibility makes structure, novelty, and evolution possible. “It is precisely through irreversible processes associated with the arrow of time that nature achieves its most delicate and complex structures” (The End of Certainty, 1997).

The step from trace to retention is the step from passive imprint to active maintenance. A fossil retains information about the past because entropy deposited it there. An organism retains information about its past because it actively sustains the conditions for retention — at thermodynamic cost.

From retention to historicity

The philosophical tradition describes the same territory from a different starting point.

Husserl identified the temporal structure of experience in On the Phenomenology of the Consciousness of Internal Time (1928). Every present moment carries what Husserl calls retention — the persistence of what just passed within present experience — and protention — anticipation of what comes next. This is a phenomenological concept, distinct from the physical retention discussed above, though both describe the carrying-forward of a past into a present. Experience is never a point; it is a flow with temporal thickness. Without retention, there is interaction but no experience.

Merleau-Ponty extended this into the body. The habitual body accumulates — skills, postures, learned responses sediment into habit. Each new perception meets a body already shaped by what came before. In Phenomenology of Perception (1945), the body is not a blank slate receiving impressions but a historical structure that carries its past forward as the condition of its present experience.

Hegel’s sublation (Aufhebung) captures the logic: every new determination preserves what it supersedes. The dialectic does not work without accumulation — it is accumulation. Nothing is simply left behind; the history of the process is carried forward in every determination (Science of Logic, 1812–16).

The affinity across disciplines is striking. Thermodynamics establishes that traces require irreversibility (Reichenbach, Albert, Rovelli). Biology shows that some systems actively sustain their own retention at thermodynamic cost (Schrödinger, Prigogine). Phenomenology describes what retention looks like from the inside — temporal thickness, bodily accumulation, the preservation of the past in every present moment (Husserl, Merleau-Ponty). Dialectics provides the logic of accumulation without loss (Hegel).

Each tradition approaches the phenomenon from its own direction. What they share is the recognition that historicity — the capacity to carry a past forward into a present — is not a given. It requires conditions, and those conditions are not universally met.

The open question

Where exactly retention begins — whether there is a threshold or a continuous gradient — remains an open question across all the disciplines involved. Prigogine’s dissipative structures suggest a gradient: the further from equilibrium, the more structured the retention. Husserl’s analysis suggests a threshold: without retention, there is no experience. Whether these are two descriptions of the same boundary or genuinely different claims is unresolved.