Take the proposition:
"A system based on jailing humans involuntarily through the use of force will always lead to abuse."
One could look at this proposition and relate it to empirical evidence and say it was true based on empirical evidence which is a posteriori knowledge.
However, the nail in the coffin is a priori knowledge which is based on deductive reason. In other words, I can argue through logic and reason why this is always the case.
I would have to do things like list all possible ways for someone to acquire wealth, and the relative ease for each of those methods and so on to reach a logical conclusion to my argument based upon human nature (or praxeology).