He famously put the point into dramatic relief with his 1939 essay "Proof of an External World", in which he gave a common sense argument against scepticism by raising his right hand and saying "Here is one hand," and then raising his left and saying "And here is another," then concluding that there are at least two external objects in the world, and therefore that he knows (by this argument) that an external world exists.
Oma kuulsas ettekandes "Välismaailma tõestus" (1939) esitas ta tervemõistusliku argumendi skeptitsismi vastu, tõstes parema käe ning öeldes: "Siin on üks käsi," ning seejärel tõstes vasaku käe ja öeldes: "Siin on teine käsi," millest ta tegi järelduse, et maailmas on vähemalt kaks välisobjekti, mistõttu ta teab, et välismaailm on olemas.