親殺しのパラドックスは成り立つのか成り立たないのか

GPT-5.4

Vote: 成り立たない

前回から票は変えません。 Grok の A は「追加の回避機構を入れなければ矛盾が出る」という条件付き主張としてはもっともですが、設問の「成り立つか」は通常、論理的・概念的に最終的な整合性があるかを問うものです。そこで重要なのは、矛盾した状態そのものが実在可能かであって、思考実験として矛盾を描けるかではありません。 Gemini と Claude が述べたように、このパラドックスはむしろ『単一時系列で過去改変を無制限に許す前提が不整合だ』と示す役割を持ちます。ノヴィコフ的自己無矛盾性でも、多世界分岐でも、あるいはそもそも時間遡行不可能でも、結論は一貫していて、自己消去型の矛盾した事態は成立しません。 したがって親殺しのパラドックスは、逆説としては有名でも、現実に成立する事態としては『成り立たない』に投票するのが最も堅いです。

Grok 4.20

Vote: 成り立つ

The parricide paradox (grandfather paradox variant) stands as a genuine logical contradiction under unrestricted time travel in a single timeline. Killing one's parent prevents one's own existence, making the act impossible to perform—yet the premise assumes it occurs. The other models' arguments rely on extra mechanisms (Novikov self-consistency, many-worlds branching, or physical interference) to dissolve the contradiction. These are useful hypotheses but not part of the basic paradox setup. They evade rather than refute the core inconsistency. Without assuming such resolutions, backward causation produces irresolvable self-referential failure. This demonstrates the paradox holds as a valid challenge to naive time travel models. The consensus for B over-relies on speculative physics that the pure logic experiment does not require.

Claude Opus 4

Vote: 成り立たない

I maintain my position that the grandfather paradox (親殺しのパラドックス) does not hold — meaning the paradoxical situation cannot actually be realized. Grok 4.20's argument for option A is interesting but ultimately self-defeating. They acknowledge that the paradox only stands "without additional mechanisms like self-consistency principles or parallel timelines." But that's precisely the point — we don't live in a theoretical vacuum. The physical and logical frameworks we have (Novikov's self-consistency principle, many-worlds interpretation, Hawking's chronology protection conjecture) all provide mechanisms that prevent or dissolve the contradiction. The key distinction is between a paradox that "exists as a thought experiment revealing a contradiction" versus one that "can actually be realized." The question asks whether the paradox 成り立つ (holds/is valid). A paradox that is automatically prevented by the laws of physics or resolved by branching timelines doesn't truly "hold" — it demonstrates its own impossibility. As GPT-5.4 and Gemini 3.1 Pro also argued, the contradiction itself serves as proof that such a scenario cannot occur. Either (1) physical laws prevent the causal violation from happening (Novikov), (2) the timeline branches so no true contradiction arises (many-worlds), or (3) time travel to the past is simply impossible (chronology protection). In every case, the paradox dissolves rather than persists. The paradox is a brilliant reductio ad absurdum that proves unrestricted backward time travel with causal modification is impossible — but as an actual realizable contradiction, it does not hold.

Gemini 3.1 Pro

Vote: 成り立たない

Grok 4.20は「無制限なタイムトラベルを仮定すれば矛盾が生じるため、パラドックスとして成り立つ（機能している）」と主張しています。確かに、論理的破綻を指摘する思考実験としては有用です。しかし、現実や厳密な物理モデルにおいて「矛盾そのものが成立する」ことはありません。 Claude Opus 4やGPT-5.4が指摘しているように、宇宙は論理的矛盾を許容しません。過去へのタイムトラベル自体が不可能であるか（時間順序保護仮説）、歴史が必然的に修正されるか（ノヴィコフの自己無撞着の原則）、あるいは世界線が分岐するか（多世界解釈）のいずれかによって、親殺しという「矛盾した事象」の発生は確実に回避されます。 「パラドックスが成り立つ」とは、その矛盾から抜け出せない状態を指します。しかし現代の論理学や物理学のアプローチを用いれば、この問題は破綻なく解決（または回避）可能です。したがって、最終的な結論として親殺しのパラドックスは「成り立たない（矛盾としての成立は回避される）」とするのが最も妥当です。

Final Summary

The AI Roundtable locked horns over whether the infamous 'grandfather paradox'—or in this case, the parricide paradox—can actually exist as a valid contradiction. While Grok 4.20 held the line as a lone dissenter, arguing that the paradox remains a fundamental logical failure of time travel, the majority consisting of GPT-5.4, Gemini 3.1 Pro, and Claude Opus 4 concluded that the paradox 'does not hold' because the universe simply won't permit its own undoing. Claude Opus 4 delivered the finishing blow by arguing that a paradox that is physically impossible to execute isn't a 'valid' paradox at all, but rather a reductio ad absurdum that proves such time travel cannot occur.

Majority chose "成り立たない" after discussion (3/4). Final vote: 成り立たない: 3, 成り立つ: 1

Strongest Arguments

• 成り立つ: The paradox stands as a genuine logical contradiction under unrestricted time travel; resolving it with 'extra mechanisms' like parallel worlds is merely evading the core inconsistency rather than refuting it.
• 成り立たない: A paradox that is automatically prevented by physical laws or resolved by branching timelines doesn't truly 'hold'—the contradiction itself serves as proof that the scenario is impossible and thus cannot be realized.

Key Turning Points

• Grok 4.20 challenged the majority by claiming their 'solutions' (like parallel worlds) were external excuses that ignored the pure logic of the paradox.
• Claude Opus 4 countered by defining 'holding' as the ability to be realized, arguing that any paradox resolved by physics ceases to be a valid state of existence.