They said therefore to him, "What then do you do for a sign, that we may see, and believe you? What work do you do?
V. Unbelief is reverse logic.  In  Mark 6:6, where it is stated that Jesus marveled because of their unbelief, the reference is to the citizens of Nazareth who rejected Jesus because he lived in their village!  This was their logic (?): We are unworthy and ignoble; Christ came from one of our families; therefore he is unworthy and ignoble!  That is exactly like saying: I hear this great and wonderful music; but since a person like I am is hearing it, it cannot possibly be true!  This is the logic (?) that supports unbelief.