The first-order axiom scheme corresponding to the introduction rule for the existential quantifier:

• • $\phi \left[t/x\right]\to \exists x\phi$

where $t$ is some term. Also called existential generalization. In free logic, the principle is challenged on the basis of apparently true statements of the form $\phi \left[t/x\right]$ that do not appear to entail that $\exists x\phi$, e.g., ‘Pegasus is a horse with wings’ does not appear to entail ‘there exists a horse with wings’. The contrapositive of the law of particularization is the law of specification.