Proto-Indo-European Roots

Root/Stem: *weid-
Meaning: to know, to see
Cognates: Greek oida (I know); idein (to see): *w disappears everywhere in Ionic and Attic dialects;
Doric Greek woida (I know)
Italic Latin videó (I see); >
Sardinian videre (to see), Aromunian vedu, Romanian vedea, Ladin vair, Italian vedere, Catalan veurer, Spanish & Portuguese ver, Occitan veire, French voir
Celtic 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)
Iranian Avestan vaeda (I know), vaédha (knowledge, information), vista (known)
Indic Sanskrit véda (I know) - from here the name Veda comes
Balkan Phrygian wit- (to know), witeto (he looked)
Armenian Armenian gitem (I know)
Germanic 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
Baltic 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)
Slavic Common Slavic *vide.ti (to see), *ve.dati (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), Upper Sorbian widźeć (to see), wjedźeć (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: Different grades of ablaut generated different meanings of the same root. In several languages the word for 'to know' acts as a Perfect tense form (like Greek oida). The explanation seems simple: 'I know' meant just 'I have seen'.