The first rule is to consider the number and variety of the
positive instances. In the case of an analogy where we have
only a single instance, the key question to ask is whether
increasing the number or variety would affect the argument.
In the analogy between reasoning and tennis, tennis is a physical
skill, but the generalization is about all skills: physical,
mental, social, and so forth. So we need to consider whether
examples from the other categories would confirm the
The second rule is to look for disconfirming instances.
In the analogy between reasoning and tennis, can you find
examples of skills that are not learned by practice? If so, this
counts against the generalization that all skills are
learned by practice.
Counteranalogies--in this case, similarities between logic and
other types of skills that are not learned by practice--are one
of the most effective ways of rebutting an argument by analogy.
The third rule is to consider the initial plausibility of a
generalization, the plausibility that there could be a connection
between subject and predicate--in this case, between S and P.
Given everything we know about skills, for example, it is quite
plausible to think that they are acquired by practice.