There are two meanings of the words possible and necessary which are of special interest to the logician more than to other men. These refer to the states of information in which we are supposed to know nothing, except the meanings of words, and their consequences, and in which we are supposed to know everything. These I term essential and substantial possibility, respectively: and of course necessity has similar varieties. That is essentially or logically possible which a person who knows no facts, though perfectly au fait at reasoning and well-acquainted with the words involved, is unable to pronounce untrue. The essentially or logically necessary is that which such a person knows is true. For instance, he would not know whether there was or was not such an animal as a basilisk, or whether there are any such things as serpents, cocks, and eggs; but he would know that every basilisk there may be has been hatched by a serpent from a cock’s egg. That is essentially necessary; because that is what the word basilisk means.