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In epistemology (theory of knowledge), a self-evident proposition is one that is known to be true by understanding its meaning without proof.

Some epistemologists deny that any proposition can be self-evident. For most others, the belief that oneself is conscious is offered as an example of self-evidence. However, one's belief that someone else is conscious is not epistemically self-evident.

The following metaphysical propositions are often said to be self-evident:

• A finite whole is greater than any of its parts.
• It is impossible for the something to be and not be at the same time in the same manner.

Certain forms of argument from self-evidence are considered fallacious or abusive in debate. For example, if a proposition is claimed to be self-evident, it is an argumentative fallacy to assert that disagreement with the proposition indicates misunderstanding of it.

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.

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.

Claims of self-evidence also exist outside of epistemology.

In informal speech, self-evident often merely means obvious, but the epistemological definition is more strict.

Moral propositions can also be said to be self-evident. For example, Alexander Hamilton cited the following moral propositions as self-evident in the Federalist No. 37:

• The means ought to be proportioned to the end.
• Every power ought to be commensurate with its object.
• There ought to be no limitation of a power destined to effect a purpose which is itself incapable of limitation.

A famous claim of the self-evidence of a moral truth is in the United States Declaration of Independence, which states, "We hold these Truths to be self-evident, that all men are created equal"; philosophically, that proposition is not necessarily self-evident, and the subsequent propositions surely are not. Nevertheless, 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 morally self-evident. Thus, as Thomas Jefferson proposed, one can hold the propositions to be self-evident as the basis for practical, even revolutionary, behaviours.

Look up self-evidence in Wiktionary, the free dictionary.

• Axiom
• Contradiction
• Foundationalism
• Self-refuting idea