In Russell’s “ + c ”, for example, “ c ” is an index which indicates that the whole sign is the addition sign for cardinal numbers. But this way of symbolizing depends on arbitrary agreement, and one could choose a simple sign instead of “ + c ”: but in “~p” “ p ” is not an index but an argument; the sense of “~p” cannot be understood, unless the sense of “ p ” has previously been understood. (In the name Julius Caesar, Julius is an index. The index is always part of a description of the object to whose name we attach it, e.g. The Caesar of the Julian gens.)