Origins
The verb 'diagnose' is a relatively recent English formation — a back-formation from the noun 'diagnosis,' which had entered English in the seventeenth century from Greek via Latin. The noun 'diagnosis' comes from Greek 'diágnōsis' (διάγνωσις), meaning 'a discerning' or 'a distinguishing,' derived from the verb 'diagignṓskein' (διαγιγνώσκειν), 'to distinguish' or 'to discern.' This verb is composed of 'dia-' (through, apart) and 'gignṓskein' (to know, to perceive), making the literal meaning 'to know apart' — to distinguish one thing from another by knowing them thoroughly.
The Greek verb 'gignṓskein' descends from the Proto-Indo-European root *ǵneh₃-, meaning 'to know,' one of the most prolific roots in the entire language family. In Greek alone, it generated 'gnṓsis' (knowledge), 'gnṓmē' (judgment, opinion), 'gnṓmōn' (one who knows, hence the pointer of a sundial), and 'anágnōsis' (reading, literally 'knowing again'). Through Latin 'gnōscere' (later 'nōscere'), the same root gave English 'cognition' (knowing together), 'recognize' (to know again), 'notion' (a thing known), 'noble' (well-known), 'ignore' (not to know), and 'note' (a mark for knowing). Through the Germanic branch, PIE *ǵneh₃- produced Old English 'cnāwan,' modern English 'know' — making 'know' and 'diagnose' ultimate cognates despite their different appearances.
The medical use of 'diagnosis' was established by the ancient Greek physician Hippocrates and his followers, who used 'diágnōsis' to mean the identification of a disease through careful observation of signs and symptoms. The Hippocratic method emphasized systematic examination — observing the patient's appearance, feeling the pulse, noting the color and consistency of bodily fluids — to distinguish one condition from another. This empirical approach to diagnosis was revolutionary in an era when illness was widely attributed to divine punishment or demonic possession.
Latin Roots
Galen, the second-century Roman physician who wrote in Greek, expanded the diagnostic vocabulary and method. His works, transmitted through Arabic translations, dominated European medicine for over a millennium. The word 'diagnosis' entered Latin medical texts directly from Greek and maintained its clinical sense throughout the medieval period.
The English noun 'diagnosis' first appeared in 1681. The verb 'to diagnose' did not emerge until 1861, surprisingly late given the noun's two-century head start. English speakers managed without the verb by using phrases like 'to make a diagnosis' or 'to determine the diagnosis.' The back-formation that created 'diagnose' followed a common English pattern: just as 'edit' was back-formed from 'editor' and 'donate' from 'donation,' 'diagnose' was stripped from 'diagnosis.'
The distinction between 'diagnosis' and 'prognosis' is etymologically elegant. 'Diagnosis' is 'knowing through' (discerning what is present), while 'prognosis' is 'knowing before' (foreseeing what will happen) — from Greek 'pro-' (before) + 'gignṓskein' (to know). A physician first diagnoses (identifies the disease) and then offers a prognosis (predicts the outcome).
Modern Usage
In modern usage, 'diagnose' has expanded beyond medicine. One can diagnose a mechanical problem, diagnose a software bug, or diagnose the root cause of a business failure. In each case, the etymological meaning holds: the diagnosis involves knowing apart — separating the actual cause from all possible causes through systematic examination.
The word 'agnostic' (one who does not know) was coined by Thomas Henry Huxley in 1869, just eight years after 'diagnose' entered the language, using the same Greek root with the negative prefix 'a-.' The temporal coincidence is fitting: the 1860s were a decade when questions of what could and could not be known — in medicine, in religion, in science — were at the forefront of intellectual culture.