In modal logic, the epistemic reading of the $4$ axiom scheme:

• • $\square \phi \to \square \square \phi$

where $\square$ is a necessity operator and $\to$ is a conditional connective. When $\square$ is given an epistemic reading as a knowledge operator, this is equivalent to

• • if $\alpha$ knows that $\phi$, then $\alpha$ knows that $\alpha$ knows that $\phi$.

This is sometimes called the $\text{KK}$ principle, and is subject to some standard counter examples.