 This validity characterizes the agent’s knowledge after the action in terms of the knowledge she had, before the action, about the effects of the action. It tells us that after the public announcement of \(\chi\) the agent will know \(\varphi\), \([\chi{!}]K\varphi\), if and only if, provided \(\chi\) could be announced, ‘\(\chi \rightarrow \)’, she knew that its truthful public announcement would make \(\varphi\) true, \(K(\chi \rightarrow [\chi{!}]\varphi)\). Note how this bridge principle, relating the two involved modalities, is not ‘chosen’: it arises as a consequence of the given definition of what knowledge is (truth in all epistemic possibilities) and the given understanding of what a public announcement does (discard all possibilities where the announcement fails).