A problem with subjective probability is that it ignores any objective fact which would make one probability “better” or “more accurate” than the other. Someone could believe a perfectly symmetrical coin has a 10% chance of coming up heads, even though such a coin is physically impossible.
The concept of subjective probability is independent of any fact about objectively existing symmetries and laws. It also ignores physical dispositions, called propensities, which is like denying that a vase is breakable because this would, allegedly, be like positing a “mysterious force” which makes it true that the vase would break if it dropped.
Subjective probability is only a measure of degree of belief, not of what a “rational” degree of belief would be, and neither is it a measure of ignorance, of how much evidence someone has about something being true or false.
It is also semantically implausible. It is perfectly valid to say “I thought the probability was low, but it was actually high. I engaged in wishful thinking and ignored the evidence I had.” But with subjective probability this would be a contradiction, it would be equivalent to saying “My degree of belief was low, but it was actually high”. That’s not what the first sentence actually expresses.
A problem with subjective probability is that it ignores any objective fact which would make one probability “better” or “more accurate” than the other. Someone could believe a perfectly symmetrical coin has a 10% chance of coming up heads, even though such a coin is physically impossible.
The concept of subjective probability is independent of any fact about objectively existing symmetries and laws. It also ignores physical dispositions, called propensities, which is like denying that a vase is breakable because this would, allegedly, be like positing a “mysterious force” which makes it true that the vase would break if it dropped.
Subjective probability is only a measure of degree of belief, not of what a “rational” degree of belief would be, and neither is it a measure of ignorance, of how much evidence someone has about something being true or false.
It is also semantically implausible. It is perfectly valid to say “I thought the probability was low, but it was actually high. I engaged in wishful thinking and ignored the evidence I had.” But with subjective probability this would be a contradiction, it would be equivalent to saying “My degree of belief was low, but it was actually high”. That’s not what the first sentence actually expresses.