 In fact, skeptical doubt about the attainability of common knowledge is certainly possible. A strong skeptical argument has been recently put forth by Lederman (2018b). Lederman builds an argument meant to undermine the possibility of deriving the common knowledge hierarchy, as done in §2, on the basis of a public event or, as Lederman calls it, public information. The principle that Lederman targets is what he calls ideal common knowledge (or belief), that is: If \(p\) is public information in a group \(G\) then \(p\) is common knowledge in \(G\), provided the agents in \(G\) are ideal reasoners. The argument rests on the privacy and interpersonal incomparability of mental states among agents, and although it is offered in terms of perceptual knowledge, its scope goes beyond perception to question the possibility of common knowledge tout court.