ABSTRACT
Deductive reasoning involves drawing conclusions that necessarily follow from some given information. For example, if I told you that Sally is shorter than Mary and that Mary is taller than Joan, you could safely conclude that Mary is the tallest of the three. However, if I asked you who was taller, Joan or Sally, you could not infer the answer from the information given. This is because the information given is consistent with three possible situations that you might represent in mental models (Johnson-Laird & Byrne, 1991) as follows:
Mary > Sally > Joan (A) Mary > Joan > Sally (B) Mary > Sally = Joan (C)
These models allow us to deduce who is tallest, but not, for example, who is shortest. Most people can solve this kind of problem, although they might need to think about it for a few seconds before answering. Consider a more complex reasoning problem, like the following from the study of Handley and Evans (2000):
You urgently need to get hold of your friend Jane. Jane is on holiday somewhere in Britain. You know she is staying in a youth hostel, but you do not know in which city. Jane, being somewhat mysterious, gives you the following information about her whereabouts:
If Jane is in Hastings, then Sam is in Brighton Either Jane is in Hastings or Sam is in Brighton, but not both
Based on this information, does it follow that: (a) Jane is in Hastings (b) Jane is not in Hastings (c) It is impossible to tell whether or not Jane is in Hastings
The reader may care to give this problem some thought before reading on.
It is possible to draw a definite conclusion that follows logically from the stated information, although it is hard to see. It involves what is called suppositional reasoning, where you need to suppose a possibility for the sake of argument. In this case, let us suppose that Jane is in Hastings and see what follows. Clearly, we can conclude from the first piece of information that Sam is in Brighton. However, the second statement tells us that either Jane is in Hastings or Sam is in Brighton but not both. So if Jane is in Hastings, by the second statement it follows that Sam is not in Brighton. So we have a contradiction. Our supposition that Jane is in Hastings has led us to conclude both that Sam is in Brighton and that he is not in Brighton. Since this is an impossible state of affairs, it follows logically that our supposition is false. Hence, we can conclude that Jane is not in Hastings.
