Category: time-and-causality
Type: paradox
Associated thinker(s): David Lewis (influential analysis)
Original publication or approximate period: Older time-travel tradition; Lewis, “The Paradoxes of Time Travel” (1976)
Difficulty: Intermediate
Themes: time travel, consistency, ability
Related experiments: The Bootstrap / Causal-Loop Paradox; The Consequence Argument
A traveler returns to a time before their parent was conceived and attempts to kill their grandfather. Assume one history, ordinary ancestry, and no substitute route to the traveler’s birth. If the attempt succeeds, the traveler seems never to exist, so cannot make the attempt. If the traveler exists and makes the trip, the ancestry must remain compatible with that existence.
A further puzzle persists even if the attempt fails: the traveler may have a working weapon, skill, and opportunity. In what sense can the traveler succeed, and in what sense can they not? The assumptions about a single history do essential work.
Does backward time travel require a contradiction, or only restrictions on what can jointly happen?
The case distinguishes logical consistency from physical feasibility. It also reveals that ordinary ability claims depend on which background facts are held fixed. Saying someone has the skills to do something is not always the same as saying that success is compatible with every fact about the wider situation.
Consistent-history response: the past can be visited but not replaced by an incompatible version of itself. An unsuccessful attempt can belong to the very history that produced the traveler.
Contextual ability response: relative to local skills and equipment, success is possible; relative to the entire actual history, it is not. Lewis argues that there is no contradiction if these senses are distinguished.
Alternative account of ability: critics contend that the relevant counterfactuals do not support saying the traveler can succeed at all. Disagreement remains about ability, even when everyone accepts that inconsistent events cannot jointly occur.
The scenario does not demonstrate that time machines exist or that general relativity permits a practical machine. A branching-world story changes the setup: killing a counterpart’s ancestor need not erase the traveler’s own history. Likewise, logical consistency does not guarantee that an attempted intervention is physically realizable.
Self-prevention stories replace the ancestor with a younger self or the construction of the machine. Branching histories offer a distinct framework. Causal loops raise another issue: a history may be consistent while leaving an event without an ordinary external origin.
The case has a broad literary and philosophical history; Lewis is credited here for influential analysis, not invention. The retrieved SEP entry supports the distinctions about ability and consistency. Physical possibility is deliberately left open.
Research check: 2026-10-30. References above were retrieved for the support described; this is an original educational explanation, not a substitute for the primary texts. Additional speculative comparisons are teaching prompts, not established historical variations.