[MeImCounting]

Ok yeah but the thing about the knife is just not true, a knifes not sharp until you de-burr it and if you just continue to grind the edge youre going to just be building a bigger burr and removing metal, eventually the knife will be ground away but thats beside the point. Good knife sharpening does only need a bit of grinding until you reach the grit you want and then de-burr but youre not going to make it dull by continuing to remove metal.

[xiaq]

That seems to be a bad translation, the original text means something like “sharpen something fully and it won’t last so long” (presumably due to it being more brittle). The text (like the rest of Tao Te Ching) is pretty vague and doesn’t actually refer to knives, so it could also be read metaphorically.

[MeImCounting]

I was being silly and pedantic but thats an interesting distinction!

I imagine Laozi probably knew how to have sharp knives so I defer to his wisdom as far as 4th century chinese knives go anyways. Perhaps they used a different technique there and then than the type of knife sharpening I'm familiar with.

[afiori]

Another interpretation is simply that you can only sharpen a knife so many times before there is no longer a knife at all

[throwanem]

Sure you will, because a knife blade good enough to hold an edge at all isn't homogeneous.

Good high-carbon steel is harder to make than mild steel. It used to be a lot harder to make; carburization in ambient atmosphere takes an enormous amount of time and fuel, and even then is hit-or-miss.

Because that's so, and also because the brittleness that makes high-carbon steel great for knife edges also makes for a very fragile blade if that's all you use, good knives historically have been made by forge-welding a strip of high-carbon steel to a larger piece of mild steel. The mild steel makes up the bulk of the blade, and the high-carbon strip is shaped and ground to form the edge.

You can sharpen that edge by removing metal, but if you do that enough, you'll remove all the high-carbon steel and end up sharpening the mild stuff. That can't take as good an edge or hold it nearly as long, so you end up with a knife that's dull no matter what you do.

This is also why sharpening steels are a thing, because good knives are usually still made this way. Stropping on a steel cleans up the edge profile without removing metal, yielding a sharper cut without shortening the life of the tool.

Granted, all of this discussion of the mechanics misses the point of the koan, which is all about moderation and humility, and nothing really to do with cleverness beyond the always wise counsel not to let it run away with you. But then, too, moderate discussion of the mechanics may in this case cut toward that end, so I don't really feel this an indulgence to excess.

[MeImCounting]

Thats a good point the knives he was familiar with were probably case hardened! So you would get to mild steel just like you said if you kept removing steel.

Nowadays though, knives are generally homogenous or some other composition like san-mai which will have the good steel in the center all the way through so even if you grind it down to a toothpick most modern knives will still have the good steel on the edge. Modern metallurgy means we can have high hardness, high toughness, high wear resistance and high corrosion resistance all in one composition. See Magnacut and other modern powder metallurgy knife steels: https://knifesteelnerds.com/2021/03/25/cpm-magnacut/

I am sure there are still some case hardened knives but for the most part case hardening is a historical technique. Same goes for the type of laminated construction you describe above.

All told the metaphor makes sense historically and in context but taken in the context of modern knives it makes less sense. This is interesting in and of itself because the meaning of metaphors and sayings can change drastically across time and social context.

[throwanem]

In this case, moderate discussion of the mechanics has disabused me of a notion! Thanks for that; I really appreciate the informative reply.

[lannisterstark]

>sharpening steels

Are you talking about honing rods? Because they 'sharpen' nothing.

[throwanem]

Sure, I guess. I don't really make a habit of pedantry, in the kitchen at least; we both evidently know very well to what tool I referred, and if it means something to you to use one name over another then you're certainly welcome to honor that preference.

[lannisterstark]

No, it wasn't about the pedantry, it was about the fact that they don't really sharpen anything. Honing rods are just used to keep the edge, generally. They won't sharpen a dull knife.