Defining ‘assumption’

In order to clarify what is meant by the word ‘assumption’ in the context of reasoning, let
us first consider what we might mean in everyday conversation by talking about ‘assuming’
something. Suppose you tell me that you are going to the post office before lunch, and I
say, ‘Take the car, because it will take you too long to walk’. You might reply, ‘You’re
assuming it will take me too long to walk, but you’re wrong’. Here you would be referring
to something which I have just stated, and telling me that I was mistaken. Hence, everyday
usage of the term ‘assumption’ can imply that an assumption is something which is
explicitly asserted, but is not, or may not be, true. One connotation of ‘assumption’, as
people normally use the word, is of a belief that we hold in the absence of strong evidence
for its truth – that is to say that the term may mark a distinction between what is known
and what is merely believed.
If we interpret the term ‘assumption’ in this way, we might think that ‘assumption’ can
refer to reasons and conclusions of arguments – that is, to things which have been stated
but which may or may not be true. However, those concerned with argument analysis
typically make a distinction between reasons, conclusions and assumptions in an argument,
and we shall be accepting this distinction here. Moreover, our use of the word will not
imply a distinction between what is known and what is merely believed.
For the purpose of our discussion of assumptions in reasoning, we shall use the word
‘assumption’ to mean something which is taken for granted, but not stated – something
which is implicit rather than explicit. It is the fact that an assumption is unstated which
distinguishes it from a reason. There may, or may not, be strong evidence for the truth of
an assumption of an argument, and this is a characteristic which it has in common with a
reason.
Sometimes in the process of evaluating arguments, the term presupposition is used instead
of assumption. We prefer the term assumption, because of the possibility of confusion
between ‘presupposing’ and ‘supposing’. Usually when arguments tell us to ‘suppose that x
is true’, they are neither stating nor assuming that x is true; they are merely exploring what
would follow from the truth of x, and often they are doing this precisely in order to show
that x must be false. So we must not take the presence of the word ‘suppose’ in an
argument to indicate that an assumption is being made. Indeed, since we are using the
term ‘assumption’ to denote something which is not stated, there are no special words in
arguments which are used to indicate the presence of this kind of assumption.
In the sense of ‘assumption’ set out above, arguments have many assumptions. For each
argument we encounter, there will be a whole host of shared background information –
for example, the meanings of the words in which the argument is expressed, and general
knowledge which gives support to the reasons which are presented. Sometimes these
assumptions will be so uncontentious that we will not be interested in making them
explicit. Sometimes, however, we will suspect that an argument rests upon a dubious
assumption, and it will be important for us to express exactly what that assumption is in
order to assess the argument.
We shall say more later about assumptions concerning the meanings of words, assumptions
about analogous or comparable situations, and assumptions concerning the
appropriateness of a given explanation. But for this chapter, we shall focus on the
following two important ways in which assumptions function in an argument; first, in
giving support to the basic reasons presented in the argument; second, as a missing step
within the argument – perhaps as an additional reason which must be added to the
stated reasons in order for the conclusion to be established, or perhaps as an intermediate
conclusion which is supported by the reasons, and in turn supports the main conclusion.
Let us explore these two uses of assumptions by looking at some examples.