Two Senses of Change

1. Review: Where We Are in Lewis’s Theory

• Earlier lectures established two pillars of David Lewis’s position on time travel:

• Logical possibility: Travel to the past is not self-contradictory if, and only if, the traveler’s actions are consistent with the history from which they depart.

• Grandfather-paradox deflation: Apparent paradoxes dissolve once we remember that the traveler’s past actions are already part of a single, internally consistent history.
  • Today’s focus: Whether and how such a traveler can really make a difference once back in the past.

2. The Worry: “Powerless Robots” or “Ghostly Observers”

• Common intuition: If a traveler’s deeds are already located at earlier external times, the traveler seems pre-programmed—a puppet unable to intervene.
• Extreme version: The traveler becomes a mere spectator, condemned to watch but never influence events.
• Lewis rejects both pictures. He insists a traveler can be:

• Concretely present in the past (a flesh-and-blood agent).

• Deliberative: holding desires, weighing options, making choices.

• Causally efficacious—but only in the correct sense of “change.”

3. Two Distinct Senses of “Change”

Lewis introduces a crucial distinction:

3.1 Replacement Change (RC)

• Core idea: One state of affairs is literally swapped out for another.
• Paradigm example:

• An intact glass falls and shatters → intact glass is gone, replaced by shards.
  • Key features:

• Applies to concrete objects.

• Implies an earlier state ceases to exist and is superseded.

• Suitable metaphor: “Erasing and overwriting” history.

3.2 Counterfactual Change (CFC)

• Core idea: We measure impact by comparing what actually happens with what would have happened had some antecedent condition been absent.
• Formally: A counterfactual conditional p \rightarrow_{cf} q (read: “If p had occurred, q would have occurred”).
• No literal overwriting occurs; history happens once. We simply note that alternative possibilities were open.

4. Why RC Cannot Apply to Times

• Times (past, present, future) are not objects that can be physically replaced.
• Illustration: Last-minute lunch reschedule

• Plan: meet at 12:00 today.

• Friend texts to shift to tomorrow.

• We did not experience two competing futures with and without the lunch, one of which was deleted.
  • Therefore:

• No replacement change is possible for any temporal location (past or future).

• RC is strictly a matter of object-level alterations, not timeline rewrites.

5. CFC in Ordinary Life

• Alarm-clock story

• Fact: Alarm sounded → speaker was on time.

• Counterfactual: \text{If (alarm fails)} \rightarrow \text{(speaker late)}.

• The day existed only in its actual version; nevertheless, the alarm mattered.
  • Battle of Waterloo

• Historical judgment (per Wellington): \text{If (Blücher late)} \rightarrow \text{Napoleon wins}.

• Blücher’s timely arrival shaped history without erasing a previously victorious France.

6. Application to Time Travel

• Lewis’s claim: A traveler can realize counterfactual change in the past while never effecting replacement change.
• Mechanism: Their causal contributions are built into the single consistent history.
• Outcome: The past is different because they were there; yet there is no second, overwritten past.

7. Illustrative Time-Travel Scenarios

• “Hitler and the Tram”

• Setup: Traveler appears in Vienna, 1908, flash startles Hitler → Hitler steps back, avoids tram.

• Counterfactual: \text{If (traveler absent)} \rightarrow \text{Hitler dies}.

• Effect: Traveler unwittingly ensures Hitler’s survival. History includes this influence once and for all.
  • “Editing the Gettysburg Address”

• Traveler meets Lincoln, who is torn between two drafts.

• Advice: Choose the now-famous “conceived in liberty” version.

• Counterfactual: \text{If (traveler silent)} \rightarrow \text{alternative speech}.

• Outcome: Traveler shapes the sole existing rendition, yet does not delete an earlier speech from the record.

8. Philosophical Pay-offs and Implications

• Agency preserved: Travelers decide and act; their freedom is compatible with a fixed, consistent timeline.
• No fatalism: Consistency ≠ inevitability. It only forbids actions that never were part of history.
• Ontological economy: No need for branching timelines or multiple histories (at least within Lewis’s single-timeline model).
• Ethical dimension: Travelers bear responsibility for outcomes they help bring about (e.g., unintentionally saving Hitler).

9. Links to Broader Lewisian Themes & Earlier Lectures

• Modal realism & counterfactuals: The same analytical machinery Lewis uses for ordinary “what-ifs” underwrites time-travel talk.
• Grandfather paradox revisited: The paradox arises only if we assume replacement change is possible for times; Lewis shows that assumption is mistaken.
• Practical upshot: Planning a time-travel mission must account for the fact that whatever you will do is already part of the past—but this still includes acts with genuine causal influence in the CFC sense.