Some sentences are self-evident. These sentences are self-evident in the same way that some concepts are self-evident. They are self-evident because they are known through the senses, by reflection or some combination of those. Sentences are true if and only if the world is the way they say it is. But all self-evident sentences are also true. Self-evident sentences are all true on pain of irrationality.
Truth is a very simple concept. Remember that sentences are a predicate that is added to a subject. If the predicate is really added to the subject in reality, then the sentence is true. Otherwise it is false. For example, a sentence might be composed of the subject “box” and the predicate “red”. If the box is red, then the sentence “the box is red” is true. Otherwise, the sentence is false.
It is irrational to believe that any self-evident sentence is false. Self-evident sentences are those that are seen by the highest standard of evidence available they are certain. There is no possible evidence available that would refute such a statement without begging the question against the self-evident truth. Furthermore, self-evident sentences are sentences that represent evidence of their own truth. Therefore, the only rational position when confronted with such sentences is to believe them. Failing to believe such sentences is irrational.
There are real sensory illusions, false memories and illogical reasoning. These things can lead to mistaken certainties. But nothing I have said about self-evidence or truth authorizes such mistakes. A detailed example will make this clear. Suppose that we are walking along a road. There is a bright camera flash and we see bright spots in our eyes. We can determine by simply moving our head that the bright spots correspond to nothing at all in reality. By that means, it is self-evident that the brights spots are spots in our eyes rather than spots in the world. Similar reasoning can used to deal with all other forms of illusion and mistaken reasoning.
The reason why it seems that illusions are something that we would believe if we believed all self-evident sentences, is that it seems that our senses must produce these sentences. But they do not. The senses on their own produce no concepts and no sentences. Without a reasoning mind, we would not have concepts at all. The mind is not a blank slate that waits for sense-experience to stamp impressions on it. Such an image is popular in the philosophical community, but there is no evidence that it is true. It is much more accurate to say that the mind examines sense-data, and immediately apprehends both concepts and truths. These concepts are self-evident concepts while the truths are self-evident sentences. Our later reasoning may produce both definitions and sentences, but these are not self-evident because they were not apprehended from sense experience.
Next, I will discuss sentences that are not self-evident and where they come from.