(quote from RealReality.org/Real_Faith_and_Reason_Vol_2_-_Scientia.pdf)

Apriorism as a Way of Knowing

Some people defend apriorism. The logical fallacy of apriorism bases reasoning on a priori thinking. Google defines a priori thinking as follows:

knowledge independent of all particular experiences, as opposed to a posteriori knowledge, which derives from experience.

Merriam Webster puts it this way:

the doctrine that knowledge rests upon principles self-evident to reason or are presupposed by experience in general.

So if you base your thinking on an axiom or presupposition, you’re reasoning by a priori thinking. You’re not proving your points using observation or experience and rational thinking. You can compare a priori thinking to a posteriori thinking. A posteriori is thinking based on experience and observation. Some philosophers teach that apriorism makes sense, and in their defense of apriorism, they assume knowledge comes out of three things:

• Reason
• What’s self-evident
• common sense

Let’s examine each of these three quickly here.

Reason: Reason must be sound, or it doesn’t result in knowledge. Sound reasoning requires a true premise and sound deductive form. A priori reasoning doesn’t have any of that, so it can’t lead to knowledge.

What’s self-evident: Who thinks it’s self-evident? What’s self-evident to one person isn’t self-evident to everyone else. Instead, whatever matches a person’s inner worldview seems self-evident to that person even if it’s false. In the same way, whatever doesn’t match a person’s inner worldview seems crazy to that person even if it’s true. Therefore, just declaring something to be self-evident doesn’t make it true. Rather, we must prove it to be true, or it’s a priori thinking.

Common sense: The term “common sense” gives the illusion of a set of commonly held beliefs. But commonly held beliefs don’t determine truth, and to imply they do is an appeal-to-common-belief fallacy.

We need to go a step further since the common sense of one person isn’t likely to be the common sense of everyone else. For instance, political liberals and political conservatives are going to have different opinions about common sense on many issues. So whose common sense are we supposed to use? Common sense is a subjective judgment. And most people determine common sense based on what matches their worldviews. What matches your worldview seems, to you, to make sense. Whatever conflicts with your worldview seems like nonsense.

And it’s not just worldviews. Peer pressure gives the illusion of universal truth since those who disagree keep quiet when they think they’re in the minority. Humans form worldviews largely under the influence of peer pressure, but worldviews are not reality. Worldviews are mere concepts of reality. Worldviews consist of made-up stuff about reality, and while they usually contain some reality and some fantasy, worldviews don’t distinguish between reality and fantasy. Worldviews make it more difficult to distinguish between reality and fantasy. For these reasons, common sense isn’t a rational basis to justify a priori thinking.

(end quote)