 The rule \(({=}\mathrm{E})\) indicates a certain restriction in the expressive resources of our language. Suppose, for example, that Harry is identical to Donald (since his mischievous parents gave him two names). According to most people’s intuitions, it would not follow from this and “Dick knows that Harry is wicked” that “Dick knows that Donald is wicked”, for the reason that Dick might not know that Harry is identical to Donald. Contexts like this, in which identicals cannot safely be substituted for each other, are called “opaque”. We assume that our language \(\LKe\) has no opaque contexts.