Page 104 - Smart Thinking: Skills for Critical Understanding and Writing, 2nd Ed

WHAT KINDS OF REASONING ARE THERE? 91

of all proceeds from the assumption that the premises are absolutely correct. If
they are, then you simply need to check that the conclusion follows logically
from them. Only then can you go back and see if there are doubts about the
premises.

Induction

In an inductive argument, unlike deduction, if the premises are true, then the
conclusion is only probably true and how big a chance that it is true depends on
the weight of evidence presented in the premises. The conclusion, then, in an
inductive argument is not guaranteed by the premises, but only supported by them.
Often, the difference expresses itself in the way that an inductive conclusion does
not state an implicit relationship but goes beyond the premises to make a new claim
altogether. Here is an example:
Imagine that, in the best traditions of the board game Cluedo, you are
conducting a murder investigation. Mr Green's body has been found
stabbed to death. In the course of your investigations, you discover
that:
Mr White says he saw Mr Black stab Green.
Black is well known to have hated Green.
Green's blood is found on Black's hands.
Ms Yellow heard Green gasp 'Black is stabbing me!'

These four claims serve as reasonably compelling evidence that Mr Black was
the murderer. However, can you be certain? No. You can only gather evidence to
increase the probability that you are correct in judging Black to be the murderer.
Indeed, if this case were to go to court, then the test used by the jury to convict or
not convict would be one of'reasonable doubt'. The jury would not have to be 100
per cent certain, simply convinced beyond a reasonable doubt that Black was guilty.
So, although you would accuse Black as if you were certain he was the murderer, in
purely logical terms you would not have deduced that conclusion from the
evidence, but have induced it and thus always be fractionally short of absolute
certainty. Remember that the claim which serves as the conclusion in this argument
'Black killed Green' is not completely implied in the premises, as we shall see ...
You, being a good detective, do some more checking and discover that:
White told people Black hated Green.
Black got bloody trying to help Green.
White disguised himself as Black to do the job.

Hence, the probability now swings around to White being the murderer. Again,
you cannot be certain, but would probably now proceed to accuse White.
Induction, then, is the process of gathering evidence and, rather than stating
something already completely contained in the premises (but not openly stated),