Form
In formal terms analogical reasoning involves argument from known similarities
between two things to the existence of further similarities. That is, one thing is
inferred to be similar to another thing in a certain respect, on the basis of the
known similarity between the things in other respects. This can be expressed in
a diagram:
Premise 1
X has attribute A
Premise 2
X is similar to Y
Conclusion
Y has attribute A
Figure 8.1 Analogy
Strength
In general, the more X is like Y, or the more X is like Y with regard to the
relevant characteristics, the stronger the argument. Hence, the strength of the
conclusion depends on the relevance and number of similarities between X and
Y.
171
As Sir Edmund Coke expressed it the similarities need to be “natural
resemblances –
that is to say real, substantial, and immersed in nature; not
merely casual or superficial, much less superstitious or exceptional”.
172
John Stuart Mill proposed a comprehensive scheme for comparing two items
with a view to formulating an analogy. In the explored regions of the items it is
necessary to identify and measure similarities and differences. What counts is
“the extent of ascertainable resemblances” compared with “the amount of
ascertained difference”. The extent of these areas of explored differences must
be compared with “the extent of unexplored region of unascertained
properties”.
173
One can also add that the nature of the similarities and
differences need to be examined as well.
Sometimes analogy has to stand on its own because it is the best reasoning
tool available. However, where one is dealing with sciences where further
inquiry and experimentation is possible the use of analogy is limited. It is “a
mere guidepost, pointing out the direction in which more rigorous
investigations should be prosecuted”.
174
___________________
171
This lead to a statement by McHugh (1999), at p 45, that a “judge who
reasons by analogy has to have an [a priori] theory that explains why the facts of
the instant case are similar or dissimilar to the facts of the precedent cases”.
172
Sir Edmund Coke Nov. Org II 27
173
John Stuart Mill A System of Logic Book III, Chapter XX
174
John Stuart Mill A System of Logic 8th ed, Longmans New Impression 1967 p
365
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