Post hoc ergo propter hoc 131
relationships is concerned with what is not true for one, the
deduction must show that the other one is also wholly or partly
excluded from some class. In other words, if they each enjoy a
different relationship with a third thing, they cannot both be in
the same class. The fallacy of drawing a positive conclusion from
negative premises persuades us that things do belong to a class
by telling of things which do not.
The trouble with this fallacy is that it can be seen coming a
mile away. You can try persuading an audience that rats are
sheep by telling them what rats are and what sheep are not. You
are unlikely to succeed for the simple reason that people smell
the rat before the wool is pulled over their eyes. It is just too easy
to spot that you cannot claim that things are the same simply
because they are different.
The only time you stand a chance of getting away with this
one is when you are calling up a radio phone-in show. And that is
only because anything goes on a radio phone-in show.
Post hoc ergo propter hocThe Latin translates as 'after this, therefore on account of this',
and it is the fallacy of supposing that because one event follows
another, then the second has been caused by the first.
Immediately after the introduction of canned peas, the illegitimate
birthrate shot up to a new high from which it did not decline until frozen
peas edged canned peas out of the market. The link is all too obvious.
(Too obvious to be true, perhaps. If your thoughts turn to feeding
your daughters beans instead, remember to keep them clear of
everything else which preceded the rise in illegitimacy. They should
stay away from television, jet aircraft, polythene and chewing-gum,
to name but a few of the more obvious hazards.)