Reasoning by analogy

Reasoning by analogy involves saying that if a number of things are similar to each other in a number of ways, they are (or should be) similar to each other in other ways as well.

The problem with reasoning by analogy lies in identifying which points of comparison between things need to be similar, and how similar they need to be, before an analogy can be considered to be sound. For example, many English people may well say to themselves: ‘We have enjoyed a number of summer holidays in some Greek islands, so we will probably enjoy a holiday in other Greek islands’. This piece of reasoning is likely to be based on a number of factors, such as the culture, the weather, the food and value for money that these people have experienced and which they are hoping to experience again. Generally speaking, this reasoning will be valid, but it needs to be more detailed if it is to be really convincing. For example, the Greek islands vary widely in terms of the quantity and quality of their classical sites. So, anyone for whom sightseeing of this sort is an important part of a good holiday needs to investigate what specific potential destinations can offer in these terms, even though such matters may be totally irrelevant to people who have other priorities while on holiday.

We will pursue reasoning by analogy in the context of legal method when we consider the doctrine of binding precedent (see pp. 104–119). However, for the moment we can simply note that lawyers accommodate the difficulty of making appropriate comparisons by saying that precedents will not be binding unless the material facts of both the earlier and later cases are the same. But before we explain this point, we must consider the other two mainstays of legal reasoning, namely induction and deduction; and more particularly, the form of deduction which takes the form of a syllogism.