The Causal Efficacy Criterion (David Armstrong famously used the term 'Eleatic principle') is one of the most popular criteria of existence. It presumes that to exist an entity must have causal effects. It may be used as a necessary, but also as a sufficient criterion.

Formulation:

An entity exists if [or if and only if, or only if] it is causally efficacious.

It may seem rather impausible to deny that Causal Efficacy is not sufficient for existence. Yet is it necessary? Can entities exist which have no causal effects?