The names are the simple symbols, I indicate them by single letters ("x", "y", "z"). The elementary proposition I write as function of the names, in the form: "fx", "φ(x,y)", etc. Or I indicate it by the letters p, q, r. 4.241    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.) 4.242    Expressions of the form "a=b" are therefore only expedients in presentation: They assert nothing about the meaning of the signs "a" and "b". 4.243    Can we understand two names without knowing whether they signify the same thing or two different things? Can we understand a proposition in which two names occur, without knowing if they mean the same or different things? If I know the meaning of an English and a synonymous German word, it is impossible for me not to know that they are synonymous, it is impossible for me not to be able to translate them into one another. Expressions like "a=a", or expressions deduced from these are neither elementary propositions nor otherwise significant signs. (This will be shown later.)