As we all know, reasons can be good or bad, better or worse. To evaluate a reason is just to assess how good or bad it is. In this topic we discuss briefly what goodness in a reason amounts to, and how you go about assessing it.

A reason is a set of claims providing evidence for some contention. The most common metaphor we use to describe this relationship is the notion of support – a reason supports a contention. Extending the metaphor, we say that a good reason provides a large amount of support, or, in our preferred parlance, strong support. Thus,

    To evaluate  a reason is to decide how strongly the reason supports

    the contention.

Truth and Relevance

There are two fundamental requirements for a reason to support a contention. First, the reason’s premises must be true. Second, they must be relevant to the contention. Consider the following:

    There are McDonald’s restaurants in both Turkey and Syria, so they will never go to war with each other.

Figure 2.25

This little argument presents a rather surprising reason for thinking that Turkey and Syria will never go to war. The reason would obviously be worthless if there were not in fact, McDonald’s restaurants in both countries. But even if there are, more would be needed. What is the connection between having McDonald’s and going to war? The information offered as evidence must be somehow relevant to the claim made in the contention. In this case the relevance is not obvious at all.

Evaluating Strength

It follows that there are two major aspects to strength of a reason. Strength is a function of the level of confidence one has in the truth of the premises, and the degree of relevance those premises have to the contention. Or, in five words or less:

    Strength = Truth + Relevance

Thus, we can break the process of evaluating a reason into three main steps:

1. Evaluate truth of premises. Assess how confident you are that the premises are true, in the light of all relevant considerations. For more on this, see Evaluating Claims. If you find that one or more premises are not true, then you can stop, since the reason has already failed a key requirement. Otherwise:

2.  Evaluate relevance to contention. Assess the extent to which the information presented in the premises makes it more likely that contention is true. A good general technique for this is scenario testing: the premises are relevant to the extent that it is difficult to think of plausible scenarios in which the contention is false, given that the premises are true.

3.  Evaluate strength. Combine the judgments you made in steps 1 and 2 to yield an overall assessment of the strength of the reason.

Degrees of Strength

Reasons vary in their strength, from offering no support at all to conclusively proving the contention. In between, there is a potentially infinite number of shades of gray. It is possible to use numerical scales to specify strength; 0 to 1, 0 to 10, and 0 to 100 have all been used. However, for practical purposes a simple qualitative scale seems to be the most workable. In this approach a reason can be worthless, weak, strong, very strong, or conclusive.

See also: Topic Reason