"perceiving with the eyes or mind, looking, beholding," c. 1300, present-participle adjective from see (v.). Seeing Eye dog is attested by 1929, American English, trademarked by Seeing Eye Inc. of New Jersey.
"action or fact of perceiving with the eyes or mind," verbal noun from see (v.). Expression seeing is believing is attested by 1660s.
"because, inasmuch as, since," used as such from early 16c., originally the present-participle of see (v.).
Then answered the iewes and sayde unto him: what token shewest thou unto us, seynge that thou dost these thynges? [Tyndale, 1526, John ii.18; where Wyclif, 1382, has What token schewist thou to vs, that thou doist these thingis?]