What is the probability to get Head at the first throw?
Since we have no further information to favor Head or Tail, we have to assume
p(Head) = p(Tail) and since p(Head) + p(Tail) = 1 we
conclude that p(Head) = 1/2. Easy as pie.
But kind of wrong, because the only information we
actually do have about the coin is that p(Head) is certainly not 1/2,
because the coin is biased.