Ex post facto - a posteriori

[Thomas Pape]

Dear Thomas S,

A posteriori relates to reasoning from effect to cause. In systematics we may apply A POSTERIORI weighting, for example giving characters weights according to how they behaved in a prior analysis - "The character has a high CI (effect!)so it must have a high phylogenetic signal (cause)".

Ex post facto is when something is done or expressed with retrospective effect. See, e.g., the American Constitution, which declares that "No bill of attainder or EX POST FACTO law shall be passed."

Regards,
Thomas Pape
Naturhistoriska riksmuseet - Stockholm