Certainty And Doubt

Certainty about doubt is doubt (1 . -1 = - 1)
Certainty about certainty is certainty (1 . 1 = 1)
Doubt about certainty is doubt (- 1 . 1 = - 1)
Doubt about doubt is certainty (- 1 . -1 = 1)

Yet there is no difference between certainty and doubt (1 - 1 = 0)