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Fig. 2.8 A diagrammatic
equivalent of syllogism
All mortalt hings.
All men.
Socrates
2.5
Deduction
Clearly deduction is one of the processes required, since once a proposition is pro-
posed it is needed to create a result from given 'facts'. The normal deductive process
can be illustrated by the syllogism:
1. All (men), are (mortal)
2. (Socrates) i sa (man)
3. Therefore (Socrates) is (mortal)
The deduction process is a formal procedure that is clearly mechanical, since it does
not involve the meaning of the words or symbols (given in brackets) when framed in
this structure. The first sentence (the 'proposition') links two phrases together such
that the first phrase 1 is said to 'contain' the second phrase 2 as a 'fact'. The first
phrase states that a general class of object (men) a share a property (mortal). The
second sentence gives an example of the general class (men) as an example of the
general class of objects. It therefore follows that this particular example (Socrates)
will have this property (mortal); after all, it has just been stated (also see Fig. 2.8 ).
Deduction contains no uncertainties and therefore does not provide any infor-
mation. During deduction a marker called the Truth-value tracks the tracing of the
certainties. The general form of this deduction is:
1. All (A) are (B).
2. (X) is a n (A).
3. Therefore (X) is a (B).
We can replace the three phrases in brackets by any other statements of facts that we
like. If the first two sentences are True after this replacement then the third sentence
will also be True since deduction preserves the marker True.
Deduction is a single step in a set of steps that will lead to conclusions that are
guaranteed to be True. Consider the following conundrum:
a. Brothers and sisters have I none
b. but this man's father
c. is my father's son
Who is he?
We can choose the following route of syllogisms:
 
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