Reasoning or knowledge which proceeds from theoretical deduction rather than from observation or experience.
Galen Strawson describes A Priori reasoning as an idea “that you can see that is true just lying on your couch. You don’t have to get up off your couch and go outside and examine the way things are in the physical world. You don’t have to do any science.” (Sommers)
Examples include “all bachelors are unmarried men” and not “all bachelors are happy”. The obvious advantage of this kind of thinking is that everyone must agree with you because what you are saying is self-evident.
The Ontological argument, although revived from time to time by various scholars (including Descartes and recent versions by Hartshorne, Plantinga and Malcolm) has never been hugely popular because it is not a very convincing way of proving God - people tend to prefer to base their reasoning on our experience of the world and this is why other arguments such as the Teleological and Cosmological have been more popular since the time of Aquinas.