Time Slice Argument

Overview

The Time Slice Argument is a family of arguments in contemporary epistemology and decision theory defending time-slice theories of rationality. According to these views, whether an agent is rational at a time depends solely on facts about that time-slice of the agent—typically, the agent’s current mental states, such as beliefs, credences, preferences, and evidence.

This approach contrasts with diachronic or history-sensitive theories, which hold that rationality at a time can depend on how the agent arrived at their current attitudes (their past evidence, earlier beliefs, or earlier choices) or on how their attitudes fit into their longer-term patterns of reasoning.

Time-slice arguments have played a central role in debates about epistemic rationality (what it is rational to believe) and practical rationality or decision theory (what it is rational to choose), especially in the context of Bayesianism, personal identity over time, and puzzles about self-locating beliefs and dynamic consistency.

Core Claims and Motivations

At the heart of the Time Slice Argument is the claim that rational evaluation is essentially synchronic. The primary target is the idea that rationality requires agents to maintain coherence across time, such as:

• satisfying diachronic Dutch book constraints in Bayesian decision theory
• updating by conditionalization on new evidence
• preserving stable preferences to avoid dynamic inconsistency

Proponents argue for several key theses:

1. Rationality-at-a-time
   Rationality is properly ascribed to an agent at a moment, not to an entire life or history. The central normative questions are:

   • Is this belief rational for you right now?
   • Is this choice rational given your current information and mental states?

2. Supervenience on the present
   If two agents are identical in all their current mental states (beliefs, evidence, intentions, preferences, etc.) and currently available options, then they should not differ in rationality. Any difference in earlier history, on this view, is irrelevant to their synchronic rational status.

3. Skepticism about historical constraints
   Time-slice theorists are skeptical of norms that say, for example, “you should now believe in a way that accords with how you believed earlier,” or “you should choose in a way that preserves your past plans.” They often regard such requirements as either:

   • merely instrumental (useful for achieving goals over time, but not constitutive of rationality), or
   • misdescribed norms that actually concern current mental states (such as current intentions about future coherence).

A central argumentative move is to consider hypothetical duplicates: Suppose there are two agents who are now mentally identical but have different pasts. If one is deemed rational and the other irrational purely because of their different histories, this seems to some to introduce an implausible kind of “normative luck.” Time-slice advocates claim this is unacceptable, and therefore rationality must be wholly grounded in the present time-slice.

Applications and Examples

Time-slice reasoning appears in several areas:

1. Bayesian Epistemology and Updating
   Standard Bayesianism holds that rational agents should:
   • have probabilistic degrees of belief (credences), and
   • update those credences by conditionalization on new evidence.

Critics of strict conditionalization sometimes use time-slice considerations to argue that what matters is whether the agent’s current credences are reasonable given their current evidence, not whether those credences can be derived from past credences by the right updating rule. An agent might have arrived at their current beliefs through a messy or “irrational” process, yet, given what they now know, those current beliefs might still be rational on a time-slice view.

1. Decision Theory and Dynamic Consistency
   In decision theory, dynamic consistency requires that an agent’s preferences and plans at different times fit together so as to avoid self-defeating or exploitable patterns (like Dutch books). Time-slice theorists question whether such cross-temporal coherence is a requirement of rationality itself, rather than merely a prudential or strategic desideratum.

For example, an agent whose preferences change over time might violate dynamic consistency but still, at each moment, be perfectly rational given their current values and information. On a time-slice view, what matters is whether the decision at each time is justified by the agent’s current mental state, not whether it lines up with their past intentions.

1. Self-Locating Belief and Personal Identity
   In debates over self-locating beliefs (beliefs about “where” or “when” one is) and branching personal identity, some philosophers (such as Caspar Hare and Adam Elga, in different contexts) have suggested that rational belief and choice should be evaluated strictly from the perspective of the agent’s present standpoint. The Time Slice Argument supports the idea that when one asks what is rational to believe “now,” one should appeal only to the information and attitudes available at that point, not to prior stages of the same person.

Criticisms and Ongoing Debates

The Time Slice Argument is controversial and has generated extensive discussion.

1. Diachronic norms and learning
Critics argue that many central aspects of rationality are inherently diachronic:

• Learning from experience: Rational belief change appears to depend on how one’s current beliefs relate to past evidence and prior credences.
• Memory and reliability: Whether it is rational to trust a memory seems to depend on a track record of accuracy, a historical property.

On this view, confining rational evaluation to a single time-slice risks ignoring essential features of rational cognition over time.

2. Responsibility and epistemic blame
Opponents contend that rational assessment often involves responsibility for past reasoning. If an agent ended up with bizarre beliefs because they deliberately ignored strong evidence in the past, many philosophers would say their current beliefs are irrational because of that history, even if, taken in isolation, those beliefs coherently fit their present evidence. Time-slice theories are sometimes criticized for obscuring such responsibility relations.

3. Planning, commitment, and agency over time
In practical rationality, long-term plans and commitments seem crucial to agency. Norms of rationality often require agents to be able to stick to their plans in the absence of new relevant information. Critics claim that a purely time-slice approach underestimates:

• the rational significance of intentions that span time, and
• the way in which rational agents govern their lives as temporally extended beings.

Proponents reply that such norms can be reconstructed as constraints on what intentions and values agents rationally have now, without positing additional history-dependent requirements.

4. Hybrid and moderate positions
Some philosophers defend hybrid views:

• Rationality-at-a-time largely depends on current mental states,
• but certain historical facts—such as how evidence was acquired, or whether there was gross past negligence—can also matter.

These positions aim to preserve the intuitive appeal of the Time Slice Argument (especially the identical-duplicate intuition) while allowing limited and principled roles for historical considerations.

The debate remains active, with ongoing work attempting to clarify:

• what exactly counts as the “time-slice” of an agent,
• which aspects of mental life (evidence, memory, intentions) are genuinely present vs. historical, and
• whether the right theory of rationality is fundamentally synchronic, diachronic, or a structured combination of both.