96 SMART THINKING: SKILLS FOR CRITICAL UNDERSTANDING & WRITING
seeks to show how one event leads to another, reasoning from generalisation shows
how knowledge about a general class or category of events allows us to make a
conclusion about a specific event that fits the general category. For example:
All children who have been fully immunised are protected against some
common and life-threatening diseases, such as whooping cough, polio,
and diphtheria. Therefore, Steven, who has been immunised, is most
unlikely to fall sick with these diseases.
The general form of such arguments is as follows:
Class X is defined by the fact that the individual cases within that class
all have property A in common; hence any individual case that is a
member of that class will also exhibit property A.So, arguing from a generalisation involves two distinct steps. First of all, it must
be established that the specific case does indeed fit the general class that is
proposed, that it is consistent. Once that 'fit' is established, then we must draw a
conclusion that relies, not on our knowledge of the specific case, but our knowledge
of the general class.
Imagine, for a moment, that you are a doctor. A woman comes to see
you with the following symptoms: swollen glands, sore throat, weakness
in the muscles, and a rash of small red spots across the back and chest.
You are not sure what is wrong with the patient but can use reasoning
to make a diagnosis: 'Almost all people who have these symptoms are
suffering from measles; this particular patient has these symptoms;
therefore she is suffering from measles'. Further, you can determine
treatment on the basis of the generalisation that 'All people suffering
from the measles need to spend a week resting in bed and take anti-
biotics to prevent secondary infections'. You know, with reasonable
confidence, that this patient has measles and so can prescribe this
treatment for her.Let us explore this form of relationship by imagining that a class is like a box
into which we put all the items that are the same as one another. On the lid of this
box are a list of requirements that determine which items can and cannot be
included. Patients who have the swollen glands and sore throat, but not the red
spots, could not be placed in the 'measles sufferer' class because they would not
meet all the requirements. Patients with the red spots but no other symptoms
would also fail to qualify (they probably have a skin irritation instead). However, as
well as a set of requirements for membership of a class or category, a generalisation
also includes a judgment that sums up the nature of those items in the category
('people with these symptoms have measles'). Hence there are two aspects to a
generalisation: a condition that determines what specific cases fit into the
generalisation, and another condition that states the common consequence or state
relating to that generalisation.