An Economic Example EXAMPLE 3
Green says:
I have to drive from Media to West Chester. I'll go by route number 403 -- In my experience, it's a fast route.
Brown says;
Don't go that way -- I read in the Daily Times that the bridge is out and route 403 is closed.
Gold says:
No, not any more -- I was that way yesterday, the bridge repairs are complete and the road is open.
The proposition under discussion here is "Route 403 is a fast way from Media to West Chester." Green, who made the claim, supports it with an undated appeal to personal experience which, by itself, is sufficient reason for rational belief in the proposition. However, Brown undercuts that reason by recent evidence from a reliable public source. Gold, in turn, undercuts the evidence from the second person by a still more recent personal observation. By destroying the credibility of the Brown's undercutting evidence, Gold restores the credibility of Green and of the proposition itself.
These examples illustrate the importance of observation and appeals to evidence in defeasible reasoning. The idea of defeasible reasoning may not sound very good -- after all, defeasible is a synonym for "fallible" -- until we realize that scientific reasoning is always defeasible, and we know the power of scientific reasoning from our everyday life. It is a commonplace of basic scientific method that experimental evidence is never quite conclusive -- it is always a logical possibility that the next experiments will go differently. Indeed, philosopher John Pollack developed the idea of defeasible reasoning largely (as I understand it) to give a stronger basis for the philosophy of science.
Since scientists are aiming at general rules, reasoning from evidence is a very strong kind of reasoning in science. Even a single failed experiment could, in principle, mean that the general rule is wrong. Example 3 is a more practical one, in which the answer may vary from one time to the next. Because of that, it is important that the reports of observation come from a reliable source (as a local newspaper, cited by Green) or better still personal (Gold) and recent (Gold). This is often a problem in economics, and even more often in business and daily life, and is one of the ways in which we cannot always be scientific in economics.
These examples also show is that argument from defeasible reasons is a bit more complex than mathematical argument, or than argument from evidence as understood by science and positive economics. When at least some of the arguments are defeasible, they must be teased out and tested by a process of give and take. The opposition has to be given its day, and if possible, shown to be wrong. Simply to assert one's position, without considering the opposition, is to take the weaker position. In order to advance knowledge, it is necessary to engage in an ongoing conversation, a conversation with rules and procedures related to its shared purposes: reasonable dialog.
