Most of this Codex makes the case for cryonics. This article is about its limits, because a procedure that pretends to work in every situation is not being honest, and honesty is the whole point of how we write here. Cryonics needs a minimum of biological integrity to mean anything. In theory anyone can sign up; in practice there are situations where preservation either cannot happen or would be too poor to deserve the name.
Too much time since death
This is the big one. Preservation has to start while the brain's structure is still largely intact, and that structure begins degrading the moment circulation stops. It is the same race against decay that the standby team exists to win. Over hours at room temperature the damage mounts; over days it becomes irreversible, taking with it the information that made the preservation worth doing. A body kept warm for too long after death has, in the terms that actually matter, already passed the point of no return.
Physical destruction of the brain
Cryonics can work around a great deal, but not the physical loss of the structure it is trying to save. Severe trauma, extensive brain injury, prolonged fire, or advanced decomposition destroy the architecture directly, and no amount of cold preserves what is no longer there. Preservation tolerates damage elsewhere in the body far better than it tolerates damage to the brain, because the brain is the part that encodes the person.
Already buried or cremated
Cremation is final in the most literal sense: it converts the structure to ash, and nothing is left to preserve. Burial is slower but no kinder to the goal; microbial activity, moisture, and time degrade tissue quickly, and exhumation even after a short period would not recover preservation-grade structure. Once either has happened, cryonics is simply off the table. A full autopsy creates the same kind of barrier for a different reason: the brain is removed and sectioned and the major vessels are cut, leaving no intact circuit to perfuse cryoprotectant through, and tissue cooled without cryoprotectant forms ice instead of vitrifying.
The avoidable failure: not being prepared
The limits above are mostly physics. This one is logistics, which means it is the one you can actually control, and the one that quietly defeats more cases than dramatic accidents do. Preservation quality depends on the team reaching you fast, and that depends on arrangements made long before the day arrives.
- Funding in place. A settled funding method lets the procedure start immediately instead of waiting on money.
- Paperwork done. Legal and medical documents completed in advance let the team act the moment death is declared.
- People informed. Family, carers, and close contacts who know your wishes, and who to call, are the difference between a timely alert and a lost window.
- Visible identification. A medical-alert bracelet or necklace tells emergency responders what you want before anyone has to go looking.
Get these wrong and a preservation that was physically possible can fail anyway, for reasons that had nothing to do with biology.
Cryonics cannot save someone whose brain structure is already gone, and it cannot beat a clock it was never allowed to start. Most of what decides whether it works for you is settled long before you die.