Because

  How to Analyze and Evaluate Ordinary Reasoning

  Section 4:  Rationales            

 G. Randolph Mayes

 Department of Philosophy

 Sacramento State University

4.1  Rationales

Reasoning is a special kind of mental activity that involves giving reasons that support or produce conclusions.  When we analyze a person's reasoning, we reconstruct it so that the logical relation between premises and conclusions becomes clear.  The result of this reconstruction is not itself a mental process, but a certain kind of logical structure we will call a rationale.

Until now we have been reconstructing reasoning into structures that look like this:

          Premise 1

          Premise 2

           .

           .

          Premise N

          Conclusion

This is the conventional mode of presentation.  But we are now going to depart from the conventional mode of representation and adopt a way of modeling reasoning  that is more graphic and more suited to the vocabulary we will be using to describe it.  A rationale looks like this:

or like this:

Both modes of presentation are acceptable.   The vertical representation  is used in our reconstruction program, Rationale 2.0.   We will typically use the horizontal alignment because it is slightly easier to use in conjunction with text.  Please note that these are the only two acceptable configurations.  In other words, do not use horizontal alignment with the conclusion on the right and the reason on the left.  Also, do not use vertical alignment with the conclusion on the bottom and the reason on the top.

A rationale is a graphic depiction of the reason supporting the conclusion.  Our convention will be to draw rationales just as you see it here, with the conclusion on the right, the reason on the left, and an arrow running from the reason to the conclusion.  The following reasoning:

• Mike has no friends because he doesn't bathe.

will be represented as follows:

A rationale with one reason supporting a conclusion is the smallest possible rationale and we can call it an atomic rationale to differentiate it from the larger ones we discuss below.

4.2  Extended rationales:  chains and branches

Rationales are capable of representing the fact that more than one reason can be given for a conclusion, and they do this quite a bit more articulately than the conventional mode we have been using so far.  To see this, consider the following examples:

1. Mike has no friends because he doesn't bathe and he is really not a very nice person.

2. Mike has no friends because he doesn't bathe an so he stinks to high heaven.

Both of these examples contain a conclusion supported by two premises.  On the conventional mode of reconstruction we would simply write the two premises above the conclusion.  But this fails to capture the fact that the premises are reasons that support their conclusions in different ways.

Using rationales, the first example is properly represented as follows:

This is what we call a branching rationale.  It represents the fact that the reasons provide independent support for the conclusion.  The second example is properly reconstructed as follows:

This is what we call a chaining rationale.  It represents the fact that "Mike doesn't bathe" is more accurately represented as the reason for "Mike stinks" rather than an independent reason for " Mike has no friends."  As you might anticipate, a combination of the two examples above:

• Mike has no friends because he stinks from not bathing and he is really not a very nice person.

would be represented in a rationale that looks like this:

Every chaining and branching rationale is just a concatenation of atomic rationales.  In the above rationale, R1' supports R1, which is to say that R1 is the conclusion of R1'.