A priori, translated very roughly but with respect to the spirit of the phrase, means “before experience”. It is used with a posteriori which means “after experience”. Something known a priori is equivalent to your prior probability; something known a posteriori is equivalent to the posterior probability. That is, when you are concerned with an event, before any experience of the event, your knowledge is a priori.
This is, of course, a slippery slope: that prior is simply the posterior of something else.
Some philosophers have tried to use a radical a priori to avoid this slope: an ideal a priori before any experience at all, a la Descartes’ “cogito, ergo sum”. This is the equivalent of the universal prior problem: what is your first prior? Unsurprisingly, their answers aren’t convincing.
However, a priori/a posteriori is still a useful distinction. The first uses some reasoning as the prior for the probability calculation; the second uses statistics as the prior. Something like existential risk requires a priori reasoning (in this limited sense) about the risks, since we can’t have an a posteriori experience about existential catastrophes.
A priori, translated very roughly but with respect to the spirit of the phrase, means “before experience”. It is used with a posteriori which means “after experience”. Something known a priori is equivalent to your prior probability; something known a posteriori is equivalent to the posterior probability. That is, when you are concerned with an event, before any experience of the event, your knowledge is a priori.
This is, of course, a slippery slope: that prior is simply the posterior of something else.
Some philosophers have tried to use a radical a priori to avoid this slope: an ideal a priori before any experience at all, a la Descartes’ “cogito, ergo sum”. This is the equivalent of the universal prior problem: what is your first prior? Unsurprisingly, their answers aren’t convincing.
However, a priori / a posteriori is still a useful distinction. The first uses some reasoning as the prior for the probability calculation; the second uses statistics as the prior. Something like existential risk requires a priori reasoning (in this limited sense) about the risks, since we can’t have an a posteriori experience about existential catastrophes.