Imperative sentences are the most obvious way to express norms, but declarative sentences also may be norms, as is the case with laws or 'principles'. Generally, whether an expression is a norm depends on what the sentence intends to assert. For instance, a sentence of the form "All Ravens are Black" could on one account be taken as descriptive, in which case an instance of a white raven would contradict it, or alternatively "All Ravens are Black" could be interpreted as a norm, in which case it stands as a principle and definition, so 'a white raven' would then not be a raven.