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We writers sometimes forget that “communication” must be logical in order to be effective. Otherwise we might as well be writing poetry (which I love and dabble with in my spare time as a hobby but never mistake it for “technical communication”).
One of the most important logical fallacies committed in technical communication is the “Fallacy of Affirming the Consequent.” It is a fallacy that we commit whole day long in daily life as well, without even being aware of it.
This is the same fallacy pointed out to by the 20th century historians of science Willard Van Orman Quine and Pierre Duhem. In academic circles it is known as the Quine-Duhem Principle of “underdetermination.”
Here is the basic outline:
STATEMENT: If A, then B.
OBSERVATION: B is correct.
CONCLUSION: Therefore A is correct.
Sound familiar and “straight forward,” doesn’t it? But unfortunately, we cannot conclude that “A is correct” just because we observe that “B is correct” since there can be an INFINITE number of reasons BESIDES A why B is correct.
If you conclude that “Therefore A is correct,” you are committing the fallacy of affirming the “consequent.” A’s truth value is always “underdetermined” since there might be other factors to determine for sure whether A is correct or not.
Here is an example:
STATEMENT: If Jim does not work hard for the test, he’ll fail the test.
OBSERVATION: Jim failed the test.
(WRONG) CONCLUSION: Jim did not work hard for the test.
Why is this conclusion wrong? Because Jim might have failed for a number of other reasons despite the fact that he did work hard for the test. Perhaps he had a splitting headache; they have changed the pre-announced test subject or level of difficulty unannounced; there has been a serious mistake in grading Jim’s test paper; etc.
The most we can say is if B is true, then A is PROBABLE. If B is true, it SUPPORTS the probability that A might be true since A also cannot be refuted. But we cannot say anything more than that.
That perhaps also explains the futility of many parents (myself included from time to time) raging at their children with that familiar phrase: “I TOLD YOU SO!”
What’s behind such an accusation? The parent claims that he/she has told the child to do X in order to prevent failure. The child did fail. So it means the original warning was true. Thus follows the “I told you so!” lecture.
But really, the child might have failed for a number of other reasons. To neglect that fact is to commit the “fallacy of affirming the consequent” or to assert the truth of an “underdetermined” hypothesis.
In technical communication, similar mishaps happen when we write passages like the following:
“If the system is configured properly 100%, then A and Z points should be aligned. Check to make sure that the A and Z points are aligned to confirm proper system configuration.”
This statement might be true 100% IF the alignment of the A and Z points is ALL that the system does.
But if the system has other functions, we cannot be certain 100% that it is perfectly configured just because A and Z are aligned. To claim so would be to commit the fallacy of “affirming the consequent.”
In real life situations this could lead to really serious problems, if for example you conclude that a nuclear reactor is working the way it’s supposed to just because one indicator yields the expected reading.
Signs of good health SUPPORTS the PROBABILITY that a person might indeed be of good health but does not and cannot guarantee it. If it did, we would never be surprised by “sudden” news of death or illness.
Watch out for relying too much on “underdetermined” generalizations. You’d be a much more rational person and a much more efficient communicator for it.