Word-A-Week in Indo-European
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Today's Word: *weid-, woid-, wid-
Translation:  to know, to see
Derivatives: Greek oida (I know) - a perfect form; idein (to see): *w disappears everywhere in Ionic and Attic dialects; 
Doric Greek woida (I know)
  Latin video (I see); > 
Sardinian videre (to see), Aromunian vedu, Romanian vedea, Ladin vair, Italian vedere, Catalan veurer, Spanish & Portuguese ver, Occitan veire, French voir
  Common Celtic *wid- (to know), *widtos (knowledge); *windo- (white, clearly seen) > 
Old Irish fís (vision), find (white), fiuss (knowledge), ro-fhetar (I know); Welsh gwyn (white), Gaulish vindos (white), Breton gwenn (white), Cornish guyn (white), 
Irish & Scottish Gaelic fios (knowledge), fís (vision), fionn (white) 
  Avestan vaeda (I know), vaédha (knowledge, information), vista (known)
  Sanskrit véda (I know) - from here "Vedas" come
Phrygian wit- (to know), witeto (he looked)
Armenian gitem (I know)
Common Germanic *wit- (to know); > 
Gothic & Old Swedish & Old English witan (to know), Old Norse vita, Old High German wizzan, Old Frankish wita; Gothic weitan (to see); 
English wise, German wissen (to know), Icelandic & Faroese vita, Norwegian vite, Swedish veta, Danish vide, Frisian witten, Dutch weten, Afrikaans weet
  Common Baltic *woid- (to know), *wid- (to see); > 
Old Prussian waidimai (we know), widdai (I saw); Lithuanian vysti (to see), Sudovian izvíst (to see), vidét (see; borrowed from Slavic)
  Common Slavic *vidéti (to see), *védati (to know); > 
Ukrainian viditi, vidati, Bulgarian vidya (I see), Macedonian vidam (I see), Serbo-Croatian vidjeti (to see), Slovene & Czech videti (to know), vedeti (to know), Slovak videt' (to see), vedet' (to know), Polish widziec' (to see), wiedziec' (to know), Lower Sorbian wjez'es' (to know), widz'es' (to see), Belorussian vedac' (to know), Russian videt' (to see), vest' (news), Old Russian vedat' (to know)
Notes: This one stem with the different grade of Ablaut (vowel interchange in stem) meant two similar things in Proto-Indo-European: "seeing" and "knowing" in the meaning of "being sure". But later the meanings were divided, as so were the different variants of the stem. If "to know" had a diphthong in stem: -ei-, -oi-, or a long -é-, "to see" was preferable with a short vowel in the root. 
The Greek verb oida, a perfect form, makes us suppose that the verb "to know - to see" used to exist in a special class of words in Early Proto-Indo-European: a class of perfect words meaning not the action, but the state, and different from verbs. Later this class of words joined the verbal system as one more tense - the perfect.
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