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 propositions are often said to be self-evident:
- A finite whole is greater than any of its parts
- It is impossible for 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, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."; 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.
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