It is a feature of much contemporary philosophical writing that an amateur of the literature finds himself unable even to think he understands it because of the extent to which the writers draw on the technicalities of modem logic. Usually such a reader does not know where to turn for enlightenment, and very frequently on being given some references to introductory books he finds himself baffled by an austere and technical exposition of the very technicalities that he wishes to understand. These few pages contain some preliminary remarks addressed only to such investigators.

Formal logic is, and so far as it has remained true to itself, always has been, an exact science. The syllogism, we once heard a theologian remark, is not an essay in vers libre. There are indeed degrees of exactness. Aristotle, who founded the science so far as concerns its European development, laid it down as a principle that phrases equivalent in significance should be interchangeable, but what phrases these might be is left to be discovered from Iris usage and forms no part of his system. We find for instance the sentence ‘all medicine is science’ treated as a substitution in the scheme for a sentence ‘B belongs to all A’ it having been stated that ‘A’ is to be replaced by ‘medicine’ and ‘B’ by ‘science’. Correct substitution produces ‘science belongs to all medicine’, and we are left to infer that Aristotle deemed this equivalent to ‘all medicine is science’. Modem standards of exactness are more demanding. A modem logician would have stated the interchangeability of ‘B belongs to all A’ with ‘All A is B’, if he desired to have both modes of expression in his system.