truism (n.)

"self-evident truth," 1708, from true (adj.) + -ism; first attested in Swift.

A truism in the strict sense (to which it might be well, but is perhaps now impossible, to confine it) is a statement in which the predicate gives no information about the subject that is not implicit in the definition of the subject itself. What is right ought to be done ; since the right is definable as that which ought to be done, this means What ought to be done ought to be done, i.e., it is a disguised identical proposition, or a truism. [Fowler, 1926]