The funny thing about trying to apply this logic in reality is that it often breaks down in ways that can be really, really bad.

I've brought up this example many times before, but Measles is a great example. Measles resets your immune system and breaks immunological memory for anywhere up to three years after having recovered from it. But now we have a bunch of people that assume any diseases can simply be dealt with in a natural way by your immune system thanks to the logic above, and well, the consequences of that are becoming clear.

No, it is not in any way a universal principle. The counterexample is Lead. A little lead in the diet does not make you stronger.

More generally regarding poisons, see https://en.wikipedia.org/wiki/Mithridatism . TLDR: YMMV.

"Mithridatism is not effective against all types of poison. Immunity is generally only possible with biologically complex types which the immune system can respond to. Depending on the toxin, the practice can lead to the lethal accumulation of a poison in the body. Results depend on how each poison is processed by the body."

"A minor exception is cyanide, which can be metabolized by the liver. The enzyme rhodanese converts the cyanide into the much less toxic thiocyanate.[12] This process allows humans to ingest small amounts of cyanide in food like apple seeds and survive small amounts of cyanide gas from fires and cigarettes. However, one cannot effectively condition the liver against cyanide, unlike alcohol. Relatively larger amounts of cyanide are still highly lethal because, while the body can produce more rhodanese, the process also requires large amounts of sulfur-containing substrates."

Our immune, metabolic, and other systems are built to be adaptable, and some things are easy to adapt to, but other things are difficult or impossible for them to adapt to.

While that deals with deliberate poisoning, when it comes to environmental contaminants such as lead and other heavy metals, or PM10s from vehicle exhausts, the other by-products of coal power stations and wood fires etc. I suspect that long-term exposure to these is not something where "you can build a tolerance" is a useful framing at all. Even if you technically do, it's irrelevant to the harm caused over time to whole populations.

There are tons of counterexamples. Chronic traumatic encephalopathy. Limb amputation. Displaced bone fractures that are never set. Crohn's Disease. Being in a coma for six months and losing all of your muscle mass. Third degree burns over 90% of your body. Plenty of things that don't kill you also don't make you stronger.

Only a sith deals in absolutes.

Nobody is suggesting you go and add some heavy metals to your corn flakes (except you).

Well they are, if they're suggesting that "what doesn't kill you makes you stronger" is anything beyond a catchy saying.

> (except you)

The post that I am responding to does in fact deal in absolutes by asserting that "What doesn't kill you makes you stronger" is a natural law. Please don't troll by attributing that to me.

My more detailed take on this is here: https://news.ycombinator.com/item?id=45653240

It is in response to someone else who is dealing in absolutes. It seems pretty common, actually. Must be a lot of Sith around today.

Sorry I might have expressed myself badly, I get it works sometimes but it's not a hard law for "everything", even if... maybe it's a good default? maybe?

Not true generally. For example, catching Measles can wipe out your immune system thereby making you more likely to get sick. Other pathogens can also work this waym

also:

https://en.wikipedia.org/wiki/Hormesis