(Explanation D): ‘If’ for Definitions Sentences in the ‘If P, Q’ form that are reasonably reversible, that is, remain true if you just reverse the P and Q, can be thought of as definitions: sentences which give the meaning of words in other words. ‘If he’s an addict, (then) he’s going to find it very hard to break the habit’, gives the meaning or definition of ‘addict’.

(b) ‘All pacifists are people who are opposed to wars’, or (Explanation D): ‘If’ for Definitions Definitions often come in a more general form than ‘If P, Q’, for example ‘All Ps are Qs’, or even, leaving out the ‘All’, as ‘Ps are Qs’. Or sometimes a definition is expressed in ‘singular’ form, namely, ‘A P is a Q’. So, (a) ‘If she is a pacifist (P), she is opposed to wars (Q)’ can be translated into: (b) ‘All pacifists are people who are opposed to wars’, or (c) ‘Pacifists are people who are opposed to wars’, or (d) ‘A pacifist is a person who is opposed to wars’.

(b) ‘All pacifists are people who are opposed to wars’, or (Explanation D): ‘If’ for Definitions So, (a) ‘If she is a pacifist (P), she is opposed to wars (Q)’ can be translated into: (b) ‘All pacifists are people who are opposed to wars’, or (c) ‘Pacifists are people who are opposed to wars’, or (d) ‘A pacifist is a person who is opposed to wars’. As a whole group, discuss which of the forms you like best – or even, whether you would use each form for a slightly different reason.

(Exercise D): ‘If’, ‘All’ and ‘A’ In pairs or groups: 1. Translate the following sentences so that you have each of them in all 4 forms: (‘If P, Q’, and ‘All Ps are Qs’, and ‘Ps are Qs’, and ‘A P is a Q’): An apprentice is a person learning a trade. All pirates are people who attack ships to rob from them. If you are a telepathist, you could read my mind. 2. Check that each of (a), (b) and (c) is ‘reasonably reversible’; in other words, it remains true when you simply swap the P and Q round.

(Exercise D, 2) Rash Reversing In pairs or groups: 3. The following appear to be definitions, but cannot be counted as such because they are not exact enough. Firstly, check that they cannot be ‘reasonably reversed’, and then discuss how they could be made more exact so as to be ‘reversible’. All picnics are meals eaten out of doors. If it is a meteorite, it has come from outer space.