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Source: p 172, A Concise Introduction to Logic (12 Ed, 2014) by Patrick Hurley

Abbreviate the Fallacy of Composition to FC and the Fallacy of Hasty Generalization to FHG.

To distinguish composition from hasty generalization, therefore, the following procedure should be followed. Examine the conclusion of the argument. If the conclusion is a general statement—that is, a statement in which an attribute is predicated distributively to each and every member of a class—the fallacy committed is hasty generalization. But if the conclusion is a class statement—that is, a statement in which an attribute is predicated collectively to a class as a whole—the fallacy is composition. Example:

[1.] Less fuel is consumed by a car than by a fire truck.
[2.] Therefore, less fuel is consumed in the United States by [the class of] cars than by [the class of] fire trucks.

At first sight this argument might appear to proceed from the specific to the general and, consequently, to commit a hasty generalization. But in fact the conclusion is not a general statement at all but a class statement. The conclusion states that the whole class of cars uses less fuel than does the whole class of fire trucks (which is false, because there are many more cars than fire trucks). Since the attribute of using less fuel is predicated collectively, the fallacy committed is composition.

I understand the first paragraph (on the general distinction between FC and HG), but I do not comprehend how 2 is a class statement. To me, 2 can be equally interpreted correctly as 3 below. I accept that 2 is likelier than 3, because 2 asserts a decrease in the mean average of fuel consumption, whereas 3 asserts a decrease in each vehicle's fuel consumption but 3 is certainly possible (For example, if suddenly a law were enacted that allowed only electric cars).

[3.] Therefore, less fuel is consumed in the United States by [ALL INDIVIDUAL] cars than by [ALL INDIVIDUAL] fire trucks.

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Both statements [2.] and [3.] are statements about the net (aggregate) amount of gas used by the vehicle classes -- therefore they are statements about the collective properties of those classes.

A hasty generalization would be more of the form "therefore for all individual cars the amount of fuel consumes is less than for any individual fire truck".

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