This observation comes from Tony Hoare’s essay “Programming is an Engineering Profession”. It offers a practical test for the simplicity of a tool: how long a manual must be to explain fully how to use it correctly, and how to avoid using it wrongly.

The idea turns an abstract quality into something that can be gauged. A tool that needs a short manual is, on this measure, simpler than one requiring a thick volume of instructions, warnings and special cases. Hoare describes it as a crude measure, acknowledging that it is a rough guide rather than an exact rule.

Applied to programming languages and systems, the point reinforces a theme found throughout his work: that simplicity aids correct use and reduces error. A tool whose complete behaviour can be described briefly is easier to understand, and therefore easier to use reliably.