The Bootstrap Paradox (Ontological Paradox)

Imagine receiving a book from your older self, preserving it for decades, then travelling back and handing that same book to your younger self. Every transfer has a local cause, but nobody writes the text. The information traces a circle rather than a line with a first source.

Fiction often calls this an ontological or bootstrap paradox. Philosophical discussions more often analyze causal loops, consistency, and explanation.

A causal loop need not contain the direct contradiction found in a story where someone both prevents and does not prevent the same event. Its pressure comes from apparently ungrounded information or objects, plus questions about entropy, wear, and the conditions for time travel.

Some accounts allow self-consistent loops while treating them as improbable or unexplained. Others argue that laws, boundary conditions, or the nature of causation rule them out.

The grandfather paradox is the other famous time-travel puzzle, and it goes wrong in the opposite way. A traveller goes back to a time before one of their parents was born and sets out to kill their own grandfather. If the attempt succeeds, that parent is never born, the traveller never exists, and nobody makes the trip, so the killing never happens. If the attempt fails, there was no contradiction to begin with. Told as a story in which the past is changed, it cannot fit into one consistent history.

The bootstrap loop has the reverse profile. Nothing in it contradicts anything else: the book is received, kept and handed back, and every event fits with every other. What is missing is an origin. The Internet Encyclopedia of Philosophy draws the line this way: the grandfather paradox is a problem of consistency, while a causal loop like the bootstrap case is a problem of origination. Philosophers divide over loops accordingly. David Lewis called them strange but not impossible; D. H. Mellor argued they are impossible; Paul Horwich argued they are implausible.

The leading responses to the grandfather paradox are different too. Lewis argued in 1976 that the traveller 'can' kill the grandfather relative to some facts (a working rifle, good aim, a clear shot) but 'cannot' relative to a wider set of facts that includes the grandfather's survival, so the apparent contradiction is an equivocation. On this view the attempt simply fails for some commonplace reason, such as a missed shot or a jammed gun. Physicists studying closed timelike curves have explored a related constraint, often called the Novikov self-consistency principle, under which only histories that are consistent all the way around a loop can occur. Branching responses send the traveller into a different history instead, which removes the contradiction but invites the objection that this is no longer travel into one's own past. A last option, in the spirit of Stephen Hawking's chronology protection conjecture, is that the laws of physics never allow the trip at all.

This page follows the dominant time-travel meaning of the search phrase. It is separate from the ontological argument for God's existence and from metaphysical paradoxes about what kinds of things exist.