2.9 Assumptions 63
Two kinds of ‘assumption’
We can see therefore that an assumption can
be explicit (stated) or implicit (unstated). This
raises an important distinction, because in
critical thinking, both kinds of assumption
play major roles. Unfortunately, in some
critical thinking textbooks, the impression is
given that ‘assumption’ always means
something unstated, and therefore implied,
whereas it is quite clear that in many if not
most arguments the premises themselves are
no more than assumptions. Unless a premise
is a known fact, the best that can be said of it
is that it is an assumption.
Take the following argument:[1] The technology for detecting forgeries has
improved in recent years. Unfortunately,
the skills and techniques of the forger are
more than keeping pace. So we are going
to see ever-increasing amounts of
counterfeit money in circulation.
The conclusion (C) is the last sentence; and the
single premise (P) is the sentence before. (The
first sentence is just context.) So, it is argued, C
follows from the explicit claim that forgery is
improving faster than detection. But what are
the grounds for that claim? We are given none.
It may be true, of course. But equally it may be
false or exaggerated. Ultimately we have to take
P on trust if we want to accept the conclusion.
It is in that sense that we treat P as an
assumption, not a fact.
But there is more to be said about [1]. For even
if we assume that P is true, it is insufficient to
establish the conclusion fully. C is a strong claim
predicting that we will see increasing amounts of
forged money. That follows from P only if the
skills and techniques of the forger continue toAn assumption is a claim or belief that is
accepted as true, even if it hasn’t been
proven or justified. Another similar word
is ‘presumption’.
We often assume (presume) something just
because there is no reason not to believe it,
even though we cannot be certain that it is
actually true. Suppose, for example, I have five
banknotes in my wallet, each for 20 euros. I
have come by them in a normal way, so I
assume they are genuine – as anyone would
unless there were some particular reason to
think otherwise. It is perfectly rational to make
this assumption because the vast majority of
banknotes we receive are genuine. Yet I know,
as well as anyone else, that some banknotes in
circulation are forgeries. Therefore, although
my assumption is a reasonable one, it is not
entirely justified; nor entirely safe. Under most
circumstances it will be true; but in others it
may be false.
This is the ordinary meaning of
‘assumption’, deriving from the verb ‘assume’.
An assumption differs from an assertion in that
an assumption doesn’t have to be stated –
although it can be. In order to make an
assertion I have to say something explicitly.
But I can make an assumption without saying
anything, or even consciously thinking it. In
fact, in the above case, I would probably give
no conscious thought whatever to the
genuineness of the notes in my wallet, unless
or until someone questioned it. My
assumption that they were genuine would be
evident in my behaviour: for example, taking
the money out to pay for something – without
a second’s thought. You could say that the
assumption I was making was implicit in
my behaviour.