I learned in a philosophy class that the opposite of "a priori" knowledge is "a posteriori" knowledge. If I said the word "chair", most people would have a priori knowledge of what an ideal chair looks like. It has four legs, a seat and a back. If I wanted a carpenter to build me a chair, however, he would use a posterori knowledge to design it. He would have the experience of seeing numerous chairs and know how to duplicate one.
Both a priori and a posteriori knowledge can be very important when it comes to a criminal trial. There's a priori knowledge that killing a person is a crime. There's also a posteriori knowledge that bringing a loaded gun to a confrontation could lead to a death. It's very similar to direct and circumstantial evidence.