Why do we say "Keep your nose to the grindstone"?

Meaning:

Apply yourself to your work with perseverance and diligence.

Background:

When sharpening blades, knife grinders tend to bend over the grindstone, or even like flat down with their faces near the grindstone in order to hold the blades against the stone.

The earliest known reference is in John Frith's A mirrour or glasse to know thyselfe in 1532: "This Text holdeth their noses so hard to the grindstone, that it clean disfigureth their faces."

Frith was an English Protestant priest who was arrested in 1532 on the orders of Thomas More (Lord Chancellor at the time) and burned at the stake a few months later for refusing to renounce his stated belief that neither purgatory nor transubstantiation could be proven.

The website phrases.org.uk notes, and dismisses, a rival explanation of this phrase as coming from the habit of millers to check that the stones used for grinding cereal weren't overheating by putting their nose to the stones in order to smell any burning. Not only is this an unlikely source as millers work millstones, not grindstones (in early English it seems that millstone and grindstone might have been interchangeable, but writings from as early as the 1400s indicate a distinction between them). Added to this, a miller occasionally sniffing the stone to check for burning does not illustrate the ethic of persistent hard work that the image of the knife grinder bent perpetually over his stone does.

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