zen_tom

Art, you are operating upon belief just as much as anyone else. Or even anything else.

Is it nonsense for a bird to believe that it's capable of flight, or does it too make 'tentative hypotheses'?

Using a word like 'nonsense' suggests you have beliefs as to what has value and what does not. It suggests that there is such a thing as 'sense'.
So, you have so far expressed the following beliefs.

1) That certainty cannot be attained or inferred from experience
2) That free-will exists
3) That some things make 'sense' and other things are 'nonsense'
4) That forming beliefs falls into the 'nonsense' category
5) That people choose to believe whatever they believe in

Or are you using the notion of belief in a less general sense? If so, please specify.