Story of Psychology - Morton Hunt [358]
For the past several decades, therefore, a number of researchers have been expanding the investigation of reasoning. Some have studied the psychological tendencies on which deductive and inductive reasoning are based; some whether either form, or some other, is what we use in everyday reasoning; some the differences in the kinds of reasoning used by experts and by novices in knowledge-rich situations. These investigations have produced a wealth of insights into the formerly invisible workings of the reasoning human mind. Here are a few of the highlights:
Deductive reasoning: The traditional idea, going back to Aristotle, is that there are two kinds of reasoning, deduction and induction. Deduction extracts a further belief from one that is given; that is, if the premise or premises are true, so is the conclusion, since it is necessarily included in them. From the premises of Aristotle’s classic syllogism
All men are mortal.
Socrates is a man.
it follows that
Socrates is mortal.
This kind of reasoning is tight, strong, easy to follow, and fully convincing. It is exemplified by proofs of logic and geometry theorems.
Yet many other syllogisms that have only two premises and contain only three terms are not so transparent; some are so difficult that most people cannot draw a valid conclusion from them. Philip Johnson-Laird, who has done research on the psychology of deduction, gives an example that he has used in the laboratory. Imagine that in a room there are some archaeologists, biologists, and chess players, and that these two statements are true:
None of the archaeologists is a biologist.
All the biologists are chess players.
What, if anything, follows from those premises? Johnson-Laird has found that few people can give the right answer.77* Why not? He believes that the ease of drawing the valid conclusion in the Socrates syllogism and the difficulty of doing so in the archaeologist syllogism are due to the way the arguments are represented in the mind—the “mental models” we create of them, a theory he has been developing and testing ever since.78
People with formal training in logic usually visualize such arguments in the form of geometrical diagrams, the two premises being represented by circles, one inside the other, or overlapping it, or separate. But Johnson-Laird’s theory, based on his research and validated by a computer simulation, is that people without such training use a more homespun model. In the Socrates syllogism, they unconsciously imagine a number of people, all mortal, imagine Socrates as related to that group, and then cast about for any other possibility (anyone outside the set—possibly Socrates). There being no such possibility, they correctly conclude that Socrates is mortal.
In the archaeologist syllogism, however, they imagine and try out first one, then another, and finally a third model, of increasing difficulty (we will spare ourselves the details). Some people rely on the first, unable to see that the second invalidates it, and others the second, not seeing that it, too, is discredited by the third and most difficult—which leads to the only valid conclusion.79
Mental modeling is not the only source of erroneous deductions. Experiments have shown that even where the form of a syllogism is simple and its mental model easy to create, people are apt to be misled by their beliefs and information. One research team asked a group of subjects whether these two syllogisms were logically correct:
All things that have a motor need oil.
Automobiles need oil.
Therefore, automobiles have motors.
All things that have a motor need oil.
Opprobines need oil.
Therefore, opprobines have motors.
More people thought the first one logically correct than did the second, although the two are identical in structure, differing only in the substitution of the nonsense word “opprobines” for “automobiles.” They were misled by their knowledge of automobiles; knowing the conclusion of the first syllogism to be true,