 which would allow the definite description to be replaced in any context by whichever defining expression fills in the ellipsis. By contrast, ∗14·01 shows how a sentence, in which there is occurrence of a description \((\atoi x)(\phi x)\) in a context \(\psi\), can be replaced by some other sentence (involving \(\phi\) and \(\psi\)) which is equivalent. To develop an instance of this definition, start with the following example: