/ˌdaɪəɡˈnoʊz/
[verb] To identify what illness, problem, or fault someone or something has by carefully examining the signs and symptoms. Doctors diagnose diseases, but mechanics and engineers can also diagnose problems with machines.
Before doctors used it, 'diagnose' was a philosopher's word — the ancient Greeks used it simply to mean 'to know apart' or 'to distinguish one thing from another.'
The story of 'diagnose' begins in ancient Greece, where thinkers were obsessed with the art of distinguishing and categorising the world around them. The word is built from two Greek parts: the prefix 'dia-', meaning 'through' or 'apart', and 'gignōskein', meaning 'to know' or 'to perceive'. Put together, 'diagnōsis' meant something like 'a knowing through' — the act of perceiving something by examining it carefully and separating it from everything else. The root 'gignōskein' itself connects to an ancient Proto-Indo-European root, 'gnō-', which is one of the oldest and most widely travelled roots in the history of language.
That ancient Greek root 'gnō-' is remarkably productive. It flowed into Latin as 'gnoscere' and 'noscere', meaning 'to come to know', and from there it branched in many directions. Latin 'noscere' gave English 'notice' and 'notion', while the prefixed form 'cognoscere' — meaning 'to know thoroughly' — passed through Old French 'conoistre' into Middle English as 'connoisseur' and 'cognition'. The same root, travelling a different path through Old English as 'cnāwan', gave us the everyday word 'know'. It is a remarkable thought that 'diagnose' and 'know' are, at their deepest level, the same word.
In ancient Greek medicine, 'diagnōsis' had a specific and important role. The great physician Hippocrates and his followers used it to describe the process of distinguishing one disease from another — a critical skill in a time when many illnesses looked dangerously similar. The Greek physician Galen, writing in the 2nd century AD, used the term extensively. The word carried real intellectual weight: a proper 'diagnōsis' was not a guess but a disciplined, reasoned conclusion drawn from close observation. This gave the word a sense of authority and precision that it has never entirely lost.
From Greek, the word passed into Medieval Latin as 'diagnosis', where it was used by scholars and early medical writers who preserved and translated ancient Greek texts. For many centuries it remained largely a learned, scholarly term — the kind of word that appeared in Latin medical treatises rather than in everyday conversation. It was used to describe the intellectual process of identifying a disease, as distinct from 'prognosis' (from Greek 'pro-', meaning 'before', combined with the same 'gignōskein' root — literally 'knowing beforehand', i.e. predicting the course of an illness).
English first borrowed 'diagnosis' as a noun in the 17th century, when educated Europeans were enthusiastically reviving Greek and Latin vocabulary for science and medicine. The verb form 'diagnose' came somewhat later, appearing clearly in print by the mid-19th century — formed by working backwards from the noun, a process linguists call 'back-formation'. This was the century of modern medicine, when identifying and naming diseases became a formal science, and the word moved from dusty Latin manuscripts into the everyday vocabulary of hospitals and surgeries. Over time, 'diagnose' has broadened far beyond medicine: today we diagnose software bugs, relationship problems, and economic crises — anywhere careful analysis is needed to name the underlying cause of trouble.