Invulnerability
Consider what "invulnerable" would have to mean if we took it seriously as a claim about a physical object.
An engineer given a component and told it is very strong does not write strong on the drawing. She wants numbers, and she wants them along several axes at once, because materials do not have a strength; they have strengths, and a material can be superb under one kind of load and pitiful under another. Cast iron takes compression magnificently and snaps in tension. Rope is excellent in tension and useless in compression. Glass is far stronger than most people imagine and fails catastrophically at surface flaws. A material that resists a slow squeeze may shatter under a sharp impact. A material that survives one impact may fail after ten thousand small ones.
So "bulletproof" is not one property. It is a claim about penetration by high-velocity small objects, and it says nothing whatever about slow crushing, about shear, about torsion, about cutting with a slow blade at high pressure, about heat, about cold, about pressure differentials, about corrosive chemicals, about electricity, about radiation, or about anything happening inside him rather than to his skin. A man who cannot be shot may be perfectly susceptible to carbon monoxide, which does not need to get through anything; it only needs him to breathe. The thing about the interior is not a quibble. A very great deal of what kills people happens without any breach of the surface at all.
And every one of these axes must be established separately, by evidence, and the evidence must be survived.
from How to Think Seriously About Superheroes (2026)
Claude Opus 5.5:
The passage's sharpest move is its last sentence, and I'd push it further: "invulnerable" isn't only many claims rather than one. It's a claim that can never be finished. A strength rating says the component survived this load, at this magnitude, for this long. "Invulnerable" says it will survive any load, of any magnitude, for any duration. That's a universal negative, and no finite set of tests establishes it. Each test confirms one point and says nothing about the next increment, the next combination of loads, or the next mode you didn't think of. Worse, as the passage notes, every test has to be survived, so gathering evidence is itself the hazard. You can only test invulnerability by trying to destroy the thing, and you get one failure.
Myth understood this long before materials science did. Baldur's mother made every thing swear not to harm him, which is exactly the passage's procedure: establish each axis separately. She skipped the mistletoe because it seemed too small to matter, and it killed him. Achilles and Siegfried each carry the one untreated patch. These stories are really Griffith's insight about glass: overall strength is set by the worst flaw, not the average material. The stories also show that enumeration fails. The threat that gets you is the one not on the list.
There's a physics problem the passage gestures at but doesn't name. Even on the one axis where the man is "bulletproof," the bullet's momentum doesn't vanish. If the skin doesn't yield, the energy goes somewhere: into the bones, the organs, the brain sloshing against the skull. Stopping a projectile at the surface just moves the damage inward. The same is true of falls. Intact skin over a body that decelerated from sixty miles an hour in a millisecond is a nice-looking corpse. The word itself carries the blind spot: in-vulnus, without wound. It defines safety as no breach of the surface, which is precisely the assumption the passage dismantles.
The deepest problem is the one the carbon monoxide example opens. A living body can't be sealed, because being alive is a matter of exchange. It has to take in air, food and water, let light reach the retina and sound move the eardrum, and shed heat and waste. Every channel through which it lives is a channel through which it can be killed. A fully invulnerable organism would be a closed system, and a closed system with a metabolism suffocates, starves, or cooks in its own heat. So invulnerability isn't just hard to verify. Past a certain point it's incompatible with being alive.
Engineering already accepts this, which is why engineers never use the word. They talk about rated loads, safety factors and failure modes, and they care a great deal about how something fails: gracefully, with warning, like ductile steel bending, or all at once, like glass. They also know the axes trade off against each other. Push hardness up and toughness usually comes down. The harder a thing is, the more brittle it tends to be. Mythic invulnerability often has exactly that shape. It's total until it isn't, and then the failure is complete.
So if we took the claim seriously, "invulnerable" would shrink into something more honest: resistant to these specified threats, up to these magnitudes, as established by these tests, with these known failure modes. That's a much more interesting property to write about, because it says where the edges are.
