If I use two signs with one and the same meaning, I express this by putting between them the sign “=”. “a = b” means then, that the sign “a” is replaceable by the sign “b”. (If I introduce by an equation a new sign “b”, by determining that it shall replace a previously known sign “a”, I write the equation—definition—(like Russell) in the form “a = b Def.”. A definition is a symbolic rule.)