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The fallacy of accident / Sweeping generalization / Dicto simpliciter


Bruce Thompson:

The argument draws a conclusion from an over-simplistic statement of a rule. This takes two forms:

Destroying the Exception by insisting on the rule, which is called Accident, and

Destroying the Rule by insisting on the exception, which is called Reverse Accident (or in Latin, Secundum Quid).

In either case, the exception falls outside the scope of the rule--or would, if the rule were stated more accurately.

Fallacy Files:


Birds normally can fly.
Tweety the Penguin is a bird.
Therefore, Tweety can fly.


The fallacy of Accident, one of Aristotle's thirteen fallacies, has been interpreted in various ways by subsequent logicians, perhaps because of the obscurity of The Philosopher's account. I will discuss only one of these interpretations here, due to its relation to recent developments in logic.

Consider the generalization "birds can fly" from the example. Now, it isn't true that all birds can fly, since there are flightless birds. "Some birds can fly" and "many birds can fly" are too weak. "Most birds can fly" is closer to what we mean, but in this case "birds can fly" is a "rule of thumb", and the fallacy of Accident is a fallacy involving reasoning with rules of thumb.