We routinely deal in logic with sentences which are true because they are negations of sentences which are false and sentences which are false because they affirm the negation of sentences which are true. In other words: not p is true if and only if p is false, and p is false if and only if not p is true.
Natural language also offers this possibility, but I haven’t made up my mind yet whether the motives for doing so are deviousness, showing off one’s verbal dexterity, or plain sloppiness. Unlike logic, English allows us to say things the way we want to, not the way we need to.
Something called an infoscreen beamed a public service announcement to weary commuters at an underground station today: ‘Latest scientific research shows that sweets negatively affect cancer of the pancreas.’ Are we to make of this that sweets halt the growth of pancreatic cancer or promote it?
Making a few replacements, we get this sentence: ‘Drought negatively affects plant growth.’ Clearly this means the slowing down of plant growth. On this logic, the announcement would seem to be encouraging people to eat sweets. But we know different, don’t we? So the reasoning should be this: plant growth is generally a good thing; cancer is a bad thing. Thus, ‘to negatively affect cancer’ means ‘to stimulate the growth of cancerous cells.’ But would you trust this kind of language if your life depended on it?
The context where these constructions are prevalent is announcements of changes in legislation and voting in general. It is common to hear about MPs approving the dropping of a proposal to send more troops to Afghanistan or, closer to home, voting in favour of rejecting the motion to withhold approval for a member of the board of my commonholders’ association. Speak in such terms to flat owners, and you will have a lot of abstentions in the room.
From another quarter, if you have the stated medical condition, your test comes back positive. If you are clear, the test is negative. Strictly speaking, we should say ‘if you test positive, you have the infection,’ because the infection is a necessary condition for the test being positive, not the other way round, but this is beside the point. The point is it is good to test negative and bad to test positive.
The logic here is easy enough to follow: if a lab technician is testing for a particular condition and fails to find it, the test result is negative, and it is good news for us. Trouble is our everyday language is elliptical and we are just as likely to say: ‘If you test negative, you don’t have the stated medical condition.’ This takes us away from the original sentence and is bound to make some people confused.
A professor I once knew would have put it down to postmodernism. Like many other endeavours in our life, our communication is also postmodern. The tape in a tape player rolls in the opposite direction to the direction of the arrows on the fastforward / rewind buttons, we turn left on a motorway in order to go right, and ticking a box means you choose not to receive advertising material rather than asking for it.