Credibility

We notice that perception as a rule is reliable. In case that was not almost always the case, we should not survive, we would be overrun by cars or even fail to bring food to our mouth.

But we also can notice that perception sometimes result in erroneous information. Maybe we thought that we saw a flash, but it was just reflection of the sun in a window just opened.

Already pre-Socratic philosophers, e.g. Parmenides (about 515 - 450 BCE) [McKirahan], established that perception cannot be claimed to be "absolutely certain" and this is still a fundamental epistemological belief.

It is hence commonly accepted that perception will never result in "absolutely certain" arguments.

Probability arguments

Arguments that are ultimately based on perception are therefore called probability arguments. The probability of the argument varies between completely unbelievable to extremely credible.

Example of "credible"

An extremely probable probability argument is "a released stone falls".

We observe gravitation at the earth's surface during every moment awake as we do not float away in space.

Gravitation seems to have the same properties within our whole solar system as seen e.g. by that humanity has succeeded in sending space probes that reach the planet Mars.

Truth / knowledge

An extremely probable probability argument is in everyday language called truth or knowledge.

Rationalistic philosophers try to claim existence of arguments that are, or may be inferred from, "absolutely certain" or "absolutely trustworthy" premises.

If such premises exist they can obviously not be ultimately created through perception (because they would then not be "absolutely certain", see above).

They therefore must have been created by another process, a priori, that not have been defined positively in a credible manner.

The philosophers have only expressed what the process is not:

... cognition independent of all experience and even of all impressions of the senses. One calls such cognitions a priori, and distinguishes them from empirical ones, which have their sources ... in experience.

Kant - Critique of Pure Reason, B2

a priori knowledge is knowledge whose justification is nonexperiential.

Casullo 2003 - A Priori Justification, p.81.