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Because How to Analyze and Evaluate Ordinary Reasoning Section 3: Reasons and Practical Implication |
G. Randolph Mayes Department of Philosophy Sacramento State University |
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3.1 Review
In the previous sections we introduced and defined the concept of logical implication as a way of articulating the rough idea that reasoning is a process of using statements we call premises to support other statements we call conclusions. But we also noted toward the end of section 2 that this idea of support is not completely captured by the concept of logical implication. We saw this in two specific ways. First, we saw that it is possible for premises to imply a conclusion, even when there is no palpable relation between these statements at all. Second, we saw that inductive reasoning seems to provide the kind of support we are talking about even though the premises of inductive reasoning fail to imply their conclusion.
Logical implication will remain a key concept for us, but in order to capture the more robust sense of support we are looking for we will need to introduce some new concepts as well.
3.2 Reasons
One way of expressing the limits of logical implication is to say that a premise can validly imply a conclusion without really being what we intuitively think of as a reason. To return to a version of a previous example, this reasoning is logically valid:
Premise 1: Birds can fly. Premise 2: Pigs can't fly. Conclusion: Birds can fly and pigs can't fly.
In fact, if you study formal logic you will learn that this valid inference is justified by a well known inference rule, sometimes called " And Introduction". Nevertheless, it would still be very strange to say something like this:
At the very least, a conclusion shouldn't just be a different way of stating the reasons. Rather, the function of a reason is to provide a legitimate way of producing a conclusion that contains information not contained in the reasons themselves. (Notice that this point also applies to our previous discussion of reasoning that is valid simply because the conclusion is a necessary truth. In such a case there is no practical connection between the reason and the conclusion, hence no sense in which the premise produces the conclusion.)
Let's define a reason as follows:
Although will not actually define what it means to produce a conclusion, we'll note this is not a deterministic notion. We are not saying that reasons produce conclusion all by themselves. They need help from other kinds of premises, and (often) from other reasons. This will become clear later.
3.3 Practical implication
Now we can introduce the concept of practical implication and define it as follows:
Although this definition requires a practical connection to exist between the premises and the conclusion, it does not require every premise to have a practical connection. So, this reasoning
Premise 1: Seth didn't want to do his homework Premise 2. Seth never does anything he doesn't want to do. Premise 3: Nimrod likes mangos. Conclusion: Seth didn't do his homework.
actually does practically imply the conclusion because it produces the conclusion using the first two premises even though the third premise is irrelevant and doesn't participate in producing the conclusion at all. We could define a stronger sense of practical implication that requires every premise to participate in the production of the conclusion, but we won't.
3.4 Two kinds of reason: cause and evidence
Exactly how do reasons produce their conclusions? This is a really important question.
Answer: A reason may contribute to the production of a conclusion in one of two ways. First, it can produce evidence for the truth of a conclusion. Second, it can produce a cause of some fact represented by the conclusion. We will discuss both of these concepts in detail in the next section, but before proceeding let's use an example to try to make the difference clear.
Suppose your friend Gwen tells you that:
Now notice that at the moment that Gwen gives you this information it will have a certain status. Specifically, you will either already possess this information or it will be news to you. Put differently, there are two distinct cognitive states you may me in prior to being told by Gwen that Seth is in jail:
Now, if you were in the cognitive state identified by (2) and you did not regard Gwen as a perfectly reliable source of information, then it would be natural to respond to this information by asking something like: Really? How do you know that? You ask this question because you remain skeptical of the information and you are looking for some further reason to believe what Gwen is telling you. A reason to believe is what we call evidence. There are many different forms of evidence, and most are quite simple. For example, Gwen could say: Seth's brother just told me.
On the other hand, if you were in the cognitive state identified by (1), then you may not require any further evidence. However, you still might want to know the the reason it happened.. The reason why something happens is what we call a cause. In this case you might respond to Gwen by saying something like: Why? What did he do? To this, Gwen might say something like: The police arrested him for shoplifting.
Although providing causes in support of a conclusion and providing evidence in support of a conclusion are two distinct act of inquiry, they have a reciprocal relationship. This is easily appreciated if we think of familiar causal and evidentiary relationships. It is because fire causes smoke, that smoke is evidence of fire. A skin rash is evidence of an allergy, because allergic reactions can cause rashes. Generally speaking, then, if A causes B, then B will also serve as evidence for A.
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