"In a strict usage the same symbol should never represent the act of sincerely asserting something and the content of what is asserted. For the symbolic distinction between the two, Frege has introduced the 'signpost' symbol. ... \vdash p is to signify the actual assertion of p, while the bare symbol p must henceforth be used only as part of a sentence. ... It should be clear from the modality of a sentence whether it is a question, a command, an invective, a complaint or an allegation of fact."