In epistemology, a self-evident proposition is one that can be understood only by one who knows that it is true. A self-evident proposition is one that can be known to be true without proof (but only by understanding what it says). Some epistemologists deny that any proposition can be self-evident.

My belief that I am conscious is considered by many to be self-evident; your belief that I am conscious is not.

Some examples of metaphysical propositions said to be self-evident include "A finite whole is greater than any of its parts" and "It is impossible for the same thing to be and not be at the same time in the same manner." Some examples of moral propositions said to be self-evident as cited by Alexander Hamilton in the Federalist #31 include "There cannot be an effect without a cause", "The means ought to be proportioned to the end", "Every power ought to be commensurate with its object" and "There ought to be no limitation of a power destined to effect a purpose which is itself incapable of limitation."

In informal or colloquial speech, "self-evident" often merely means "obvious."

Certain forms of argument from self-evidence are considered fallacious or abusive in debate. An example is the assertion that since an opponent disagrees with a (claimed self-evident) proposition, that he must have misunderstood it.

Compare with: the concepts of primitive notion and axiom in mathematics.

It is sometimes said that a self-evident proposition is one whose denial is self-contradictory. It is also sometimes said that an analytic proposition is one whose denial is self-contradictory. But these two uses of the term self-contradictory mean entirely different things. A self-evident proposition cannot be denied without knowing that one contradicts oneself (provided one actually understands the proposition). An analytic proposition cannot be denied without a contradiction, but one may fail to know that there is a contradiction because it may be a contradiction that can be found only by a long and abstruse line of logical or mathematical reasoning. Most analytic propositions are very far from self-evident. Similarly, a self-evident proposition need not be analytic: my knowledge that I am conscious is self-evident but not analytic.

That being said, an analytic proposition, however long a chain of reasoning it takes to establish it, ultimately contains a tautology, and is thus only a verbal truth--a truth established through the verbal equivalence of a single meaning. For those who admit the existence of abstract concepts, the class of non-analytic self-evident truths can be regarded as truths of the understanding--truths revealing connections between the meanings of ideas.

One of the most famous examples of claims to the self-evidence of a truth is found in the Declaration of Independence. The proposition that "all men are created equal" is not necessarily self-evident in a philosophically respectable sense, and the propositions which follow surely are not. However many would agree that the proposition "we ought to treat subjects known to be equal in a certain sense equally in regard to that sense" is self-evident. On the other hand the propositions described can be (as Thomas Jefferson proposed) "held" to be self-evident as the basis for practical, even revolutionary, behaviors.