In the branch of Philosophy known as Epistemology (study of Knowledge) there has been the traditional view dating back to Plato the Knowledge is a Justified-True belief. (JTB account of Knowledge). In other words, to have epistemic rights to believe something, one needs to have justification and for it to be knowledge it must be a belief that is true. What counts and doesn’t count as justification for Knowledge?
Well one way is from our experiences and observations including our senses. This is includes human experience and scientific discovery. A popular scientific theory is the germ theory of disease. This has been observed more years and years and held to be plausible and credible. Therefore, science counts as justification for a belief. Along with that, human experience counts as justification as well. From experience we know that there is an external world outside of ourselves by our 5 senses. One is justified in believing what they see on a day to day basis or event to event, unless someone has a reason to doubt their senses, which would be things such as a person prone to hallucinations.
The other justification of knowledge comes through reason alone. In other words, through reason we can have justification for beliefs. Things that are pure knowledge are things that are true by definition like, “There are no married-bachelors”. Then there are rational axioms that we derive our from which we can derive knowledge from and thus can count as self-justification with no further justification required. In a strict sense, self-evident principles include the formerly mentioned tautologies or things true by definition. Furthermore, things like “I exist” are self-evident because if one is saying this or thinking this then they must necessarily exist. In addition, the principle of non-contradiction is something that is self-evident. If something is a contradiction, then it is necessarily false. For example, I exist and not exist at the same time. This is not possible and absurd.
Another self-evident example is known as the law of identity. This means that something is what it is and it’s not what it’s not. So, a square is what it it is and not what it’s not. A square is not a circle by definition and it has four sides. When it comes to what counts as self-evident or self-justifying it seems to me that it’s better to be more strict than liberal. After all, we don’t want to say everything is self-evident because that’s obviously false, and this would lead to the end of rational discourse and ultimately leads to dogmatism and close-mindedness.