Let's imagine a family, let's say Betty, her husband Barney, and their
son Bamm-Bamm. Bamm-Bamm gets caught stealing money from Betty's
purse. Then he gets caught shoplifting candy from the local Wal-Mart.
Next he is found to have purloined some of his friend Pebbles' toys.
Betty and Barney conclude that little Bamm-Bamm is a "thief."
Nevertheless, when Pebbles' mother Wilma suggests they take Bamm-Bamm
to a psychotherapist, Betty and Barney do so.
The psychotherapist examines Bamm-Bamm and explains that Bamm-Bamm is
stealing things because of some kind of psychiatric condition, and
recommends both individual and family therapy. Betty and Barney
agree. They go home, trying to adjust to the finding that Bamm-Bamm
is actually an "emotionally disturbed thief."
To be a "thief," of course, is a type of profession. Call him a
"thief" enough and Bamm-Bamm is likely comply with his parents' choice
of career for him.
What is Betty and Barney's error in reasoning? What ought the family
therapist teach them?
Here's the error. Having already reached the conclusion that
Bamm-Bamm is a "thief," Betty and Barney take the new data that
Bamm-Bamm is emotionally disturbed and tack it onto their prior
conclusion. Bamm-Bamm, they believe, is an "emotionally disturbed
thief." This is, generally speaking, an incorrect form of reasoning,
like doing addition when the arithmetic calls for multiplication
instead. When important new information becomes available, prior
conclusions should be dropped, at least in general, because they are
based on the implicit assumption that the newly discovered information
was irrelevant. The correct conclusion in this case is that little
Bamm-Bamm has been stealing because of an emotional disturbance, which
is what the psychotherapist told Betty and Barney.