The two terms describe the foundations upon which we know something (a proposition) is knowable or true.
A proposition is knowable a priori if it can be known independent of any fact or experience apart from the experience of the language in which the proposition is made. E.g., the proposition that all widows were once married is knowable a priori (you only need to know what a 'widow' is).
A statement is knowable a posteriori only if our experience is needed to provide some additional information for it to be affirmed or denied. E.g., the claim that 'the sun is shining' cannot be known to be affirmed or denied without looking out the window.
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