As a first step in analysis, we can represent the structure of an argument by analogy as follows:

1.  A and B are similar

2.  A has property P
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3.  B has property P

A and B are two things being compared: skill in tennis and the art of reasoning.

The conclusion is that B has a certain property: the art of reasoning must be acquired by practice.

The argument is that B has this property because it is similar to A, which has the property. The art of reasoning must be acquired by practice because it is similar to tennis in being a skill.

A premise about tennis can yield a conclusion about logic only on the assumption that tennis and logic are similar.

If two things are similar, they must be similar in some particular respect--in shape, color, function, or whatever. To put it differently, two things are similar because they share some property. So the first task is to identify the respect in which A and B are similar, to identify the property they have in common. In some cases, this property is stated explicitly, but in others it isn't.

In the argument about tennis and logic, the property is stated explicitly: they are both skills.

We'll use the letter S to stand for the property that A and B have in common, the property that makes them similar. We can reformulate the first premise in an argument by analogy as follows:

1.  A and B have property S

2.  A has property P
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3.  B has property P

We can go on to ask the next--crucial--question. What is the relationship between S and P? If there is no connection between these two properties, then the conclusion does not follow.

So the strength of the argument depends on the likelihood of a connection between the properties involved, and our goal in evaluating an argument by analogy is to estimate this likelihood.