In a nutshell: "I don't see any evidence for X, therefore I am going to hypothesise that X does not exist, until evidence to the contrary appears."

Quite reasonable. But as you asked, Linear, the question then becomes whether you are willing to change your views when evidence to the contrary does appear.

You have to be careful about the first part of the statement above, however. If there is the possibility that X exists, there may never be any evidence uncovered until someone goes to look for it.