It seems like in order to go from P(H) to P(H|E) you have to become certain that E. Am I wrong about that?

Say you have the following joint distribution:

P(H&E) = a

P(~H&E) = b

P(H&~E) = c

P(~H&~E) = d

Where a,b,c, and d, are each larger than 0.

So P(H|E) = a/(a+b). It seems like what we're doing is going from assigning ~E some positive probability to assigning it a 0 probability. Is there another way to think about it? Is there something special about evidential statements that justifies *changing *their probabilities without having updated on something else?