The English word ‘reason’ is used in many different ways; when used as a noun, it can refer to a number of different kinds of things. However, when studying reasoning and argument, we are mostly interested in just one of these kinds of reasons. In this sense,

    A reason is a group of claims, known as premises, treated as jointly

    providing evidence that another claim is true.

Reasons of this kind are logical or evidential reasons. These contrast with another large and important category of reasons, the explanatory reasons or explanations.

Note that although a reason is a group of claims working together, often in ordinary dis- course only one of those claims is actually presented (see enthymeme). In such situations people often refer to that one claim as the reason, though strictly speaking it is only part of the full reason.

Reasons and Evidence

What is ‘providing evidence that another claim is true’? The basic idea is that, if the reason is any good at all, the premises make it more likely that the contention is true. We talk about this special relationship in a variety of ways:

  - A reason supports the contention.

  - A reason lends weight to the contention.

  - A reason lends credence to the contention.

  - A reason provides a basis for believing the contention.

  - A reason proves the contention (if it is very good).

  - You can infer the contention from the reason.

Figure 2.7

Good Reasons and Bad Reasons

What makes a group of claims a reason? Answer: being treated as evidence relating to that claim. To be a reason is to be sincerely put forward or accepted as a reason by somebody.

This has a very important consequence: reasons can be bad. The person advancing the reason probably thought it was pretty good, but may have been confused about that.

So when is a reason a good one? There are two main dimensions to the quality of a reason. To be good, a reason should be

  - Well-formed. The reason must be built correctly. The premises must relate to the conclusion and to each other in the right kind of way. For more on this, see below.

  - Strong. The reason must be such that make the conclusion substantially more likely to be true than it would be in the absence of that reason.

These dimensions are not independent of each other. In order to have strength, reasons must be well-formed. Still, it is quite useful to think of reasons from these two distinct perspectives.

See also: Topic Evaluating Reasons