Mathematicians of every age have been seen to criticize the proofs of their predecessors or of their contemporaries as ‘not being rigorous’; and often those which they proposed as replacements for the defective proofs were in their turn considered inadequate by the succeeding generation. This apparent repeated putting into question of mathematicians' arguments at length led some people to think these arguments contained a hidden flaw which meant that a proof could never be found unassailable by any criticism.