This chapter provides a sample of the criticisms of philosophers on the course of linguistic philosophy. In common speech the word 'necessary' means much the same as 'indispensable', 'inevitable', 'without any possible alternative'. If words are to be taken in their ordinary senses, it seems obvious that necessary truths cannot be true by convention. Admittedly the phrase 'necessary truth' belongs to philosophers' jargon rather than to common speech, but its origin is clear enough. Philosophers say that a proposition is a necessary truth if it is impossible that it should not be true, i.e. if there is no possible alternative. But in common usage the phrase 'by convention' always implies the possibility of an alternative. Some philosophers who profess conventionalism may be under the impression that they can vary the rules of usage of the symbols without altering the meaning of those symbols.