The second law of thermodynamics and the psychological arrow of time

Meir Hemmo, Orly Shenker

Research output: Contribution to journalArticlepeer-review


Can the second law of thermodynamics explain our mental experience of the direction of time? According to an influential approach, the past hypothesis of universal low entropy (required to explain the temporal directionality of the second law in terms of fundamental physics, which is time-symmetric) also explains how the psychological arrow comes about. We argue that although this approach has many attractive features, it cannot explain the psychological arrow after all. In particular, we show that the past hypothesis is neither necessary nor sufficient to explain the psychological arrow on the basis of current physics. We propose two necessary conditions on the workings of the brain that any account of the psychological arrow of time must satisfy. And we propose a new reductive physical account of the psychological arrow of time compatible with time-symmetric physics, according to which these two conditions are also sufficient. Our proposal has some radical implications, for example, that the psychological arrow of time is fundamental, whereas the temporal direction of entropy increase in the second law of thermodynamics and the past hypothesis is derived from it, rather than the other way around.

Original languageEnglish
Pages (from-to)85-107
Number of pages23
JournalBritish Journal for the Philosophy of Science
Issue number1
StatePublished - Mar 2022

Bibliographical note

Funding Information:
We thank David Albert, Heather Demarest, and Barry Loewer for comments on issues in this article. Also, we thank two anonymous reviewers for raising valuable points and questions. This research has been supported by the Israel Science Foundation (grant number 1148/2018).

Publisher Copyright:
© The Authors. Published by The University of Chicago Press for The British Society for the Philosophy of Science.

ASJC Scopus subject areas

  • History
  • Philosophy
  • History and Philosophy of Science


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