74 SMART THINKING: SKILLS FOR CRITICAL UNDERSTANDING & WRITING
conclusion is obvious. In fact, from the point of view of the customer, it is not, and
thus the reasoning used by the technician is ineffective.
By definition, all reasoning depends at some point or another on assumptions that
give rise to implied claims. So, practically speaking, effective reasoning does not require
that there be no implied premises. But it does require that we be well aware of the
claims that we do leave out. First, if we do not recognise our own implied premises,
then we may fail to judge accurately if they are true or not; second, we may fail to
communicate our message to someone else who does not share our assumptions. This
last point is particularly important. Our decisions about using implied premises can
only be guided by what we expect our audience to know, and what we know, about
the context of our reasoning. For example, academic essays and reports are usually
designed precisely to test students' abilities to avoid making assumptions, and so, we
would not want to leave many implied premises in this context, even though we might
assume our audience (the assessors) do know the claims we are making.
Exercise 6.1
Think of two completely separate reasons for each of the following two con-
clusions. Write these reasons down, and then analyse their relationship to the
conclusion, expanding each into a chain of dependent premises. In the first case,
you are explaining why the conclusion is happening; in the second case you are
arguing for it to be accepted.
a. I am reading a book on reasoning.
b. There are considerable benefits to be gained from studying how to think
better.
Relevance
What is relevance?
Here is a simple example of relevance and irrelevance concerning the conclusion
'Smith is physically unhealthy':
a. Smith has pains in his chest; he coughs a lot and is short of breath
walking up stairs. Clearly Smith is physically unhealthy.
b. Smith wears green trousers and a pink hat and has no shirt on. Clearly
Smith is physically unhealthy.
In argument a, the relevance of the premises is clear: they all report physical
symptoms that are routinely recognised as evidence of poor health. In the second
case, these premises are irrelevant because they give us no indication of physical
health. Note that it is impossible to determine the relevance of the premises by
themselves: we must look at their relationship to the conclusion. Argument b
contains a number of irrelevant premises, but if the conclusion were 'Smith has no