These lines illustrate Gloucester's failing. When he had his vision, he could not see the deceit fabricated by his younger son, and thus, vision has not helped him see his way in the past. Now that he has lost his vision but finally seen the truth, Gloucester can envision no way in which he can regain the elder son, who is lost to him. For Gloucester, the disadvantage of lost sight has become an advantage (IV.1.20-21), and his only wish is that he might "live to see thee [Edgar] in my touch" (IV.1.23).