WHAT KINDS OF REASONING ARE THERE? 93
else will also occur. It is, perhaps, the philosopher's version of Newton's third law
of thermodynamics, which stated that all actions have an opposite and equal
reaction. Let us have a look at an example which uses a series of if/then statements
to prove that Australia's economic health depends, not on low wages, but on high
wages:
- If Australia's wages are reduced, then people will have less to spend.
- If people have less to spend, then consumption will fall.
- If consumption falls, then the economy will slow down.
- If the economy slows, then business profits will fall.
- Therefore, if we want to avoid a loss in profit, we must not reduce
wages.
The power and flexibility of propositional logic is demonstrated by this
example, not because these premises guarantee the conclusion is true, but rather
because they create a series of logical relationships between two otherwise
apparently unconnected events—the need to avoid a loss in profit and the desire
not to reduce wages. If we were then to set about convincing someone of this
ultimate conclusion, we would, by having set up the chain of propositions in this
manner, have identified the key sub-conclusions that would each need to be
supported by sub-arguments. Thus, we would have to establish that it was indeed
reasonable to believe that 'If consumption falls, then the economy will slow down',
and we might do this by reference to real-world examples such as previous
economic conditions in which a fall in consumption has indeed caused an
economic slowdown.
The lesson to learn here is: while categorical logic concerns itself with the
structural relationship of the categories we use, defining the inclusions and
exclusions so that we can be sure what does or does not belong together as a group,
propositional logic prompts us to ask the right questions about what we need to
establish, inductively, to then make our overall argument convincing.
Five types of reasoning
It is important to recognise that these five types are not mutually exclusive. We
will consider causal reasoning but, for example, we also see that when looking
at causes we are also asserting analogies between the cause of one event and
another. Equally, when we look at analogies, there are ways in which analogical
reasoning is the same as reasoning from generalisations or might involve causal
relationships. Thus, the five types presented here are not done so in the same
manner as the discussion of deduction/induction which showed how arguments
of one type (in each case) could not be of the other type. Rather, I present these
five types to assist you in thinking more broadly about the kinds of questions
you might ask in your reasoning and (as we will see in chapter 8) to guide your
search for information.