Buried in the preface of a 1902 trade manual for photoengravers — a workshop guide for men who etched zinc plates with acid to reproduce illustrations for newspapers and magazines — Harry Jenkins makes a quietly radical argument about what it means to be a skilled tradesman. The book itself is a practical recipe book: how to coat a plate, how to mix the bichromate sensitizer, how to expose a halftone screen. But before the formulas, Jenkins pauses to deliver a small sermon:

The importance of a study of the scientific laws upon which the practical work is based can not be too strongly emphasized. It is the possession of this knowledge that makes the difference between the intelligent investigator and the "rule-of-thumb" workman, and the student is urged to give ample attention to these fundamental principles.

This is a striking framing for 1902. The photoengraving trade was, at the time, the digital photography of its day — a young, fast-moving technology that had only existed commercially for about two decades. Most practitioners learned by apprenticeship, copying the master's gestures without understanding why the chemistry worked. A "rule-of-thumb" workman could produce a perfectly serviceable plate on a good day, but when the bath went off, or the weather shifted, or a new pigment behaved oddly, he had no theory to fall back on.

Jenkins is arguing for what we'd now call first-principles thinking. He wants his readers to understand the chemistry of silver halide reduction, the optics of the halftone screen, the acid-resist behavior of hardened gelatin — not because the theory is beautiful, but because recipes break and principles don't.

What's remarkable is how thoroughly modern society has re-learned this lesson and then forgotten it again. The contemporary parallel is almost too obvious: the developer who can wire up a React component by pattern-matching against Stack Overflow but cannot debug when the framework misbehaves; the data scientist who runs model.fit() without understanding gradient descent; the home cook who follows recipes flawlessly but cannot rescue a broken sauce. Jenkins would recognize all of them as "rule-of-thumb workmen."

The forgotten wisdom here isn't a technique or a recipe — it's an attitude toward craft. In an era when YouTube tutorials and LLM-generated code make the rule-of-thumb path easier than ever, Jenkins's 1902 preface reads like a warning shot across 124 years. He believed that the difference between a competent tradesman and a master wasn't dexterity or experience, but the willingness to learn the underlying science when the trade school told you that you didn't need to.

The photoengraving industry that Jenkins wrote for was obliterated by offset lithography within fifty years. But the practitioners who understood the chemistry — the Frederic E. Ives who co-wrote Jenkins's color-work chapters — went on to invent the next generation of imaging. The rule-of-thumb workmen simply lost their jobs.