Self-study
Support If the law punishes littering, then the city has an obligation to provide trash cans. ███ ███ ███ ████ ███ ██████ ██████████ ██ ███ ████ ███ ██ ████ ███████████
Method of Reasoning
The author identifies a conditional relationship (if littering punished, then provide trash cans). He negates the sufficient condition (littering isn’t punished), and assumes the necessary condition is also negated (don’t have to provide trash cans).
Identify and Describe Flaw
The author’s reasoning is flawed, because negating a sufficient condition does not automatically justify negating a necessary condition.
For example, if something is an apple, then it’s a fruit. But it doesn’t follow that, if something is not an apple (e.g. an orange), then it’s not a fruit. This unsound inference would be analogous to the stimulus.
21.
Which one of the following ████████ █ ██████ ███████ ██ █████████ ████ ███████ ██ ████ ██ ███ ████████ ██████
a
If today is █ ████████ ████ ███ ██████ ████ ███ ██ █████ ███ ██████ ██ ███ ████ ███ █████████ ████ █████ ██ █ ████████
This is the wrong flaw. (A) takes a conditional statement and erroneously assumes that it’s still valid when reversed. By contrast, the stimulus takes a conditional statement and mistakenly assumes that it’s still valid when negated.
b
Jenny will have ████ ██ ████████ ██ ███ ████████ ██████ █████ ███ ██ ████████ ██████ ████ ██ █████ ██ ███ ███ █████████
This is the wrong flaw. (B) is close to the valid contrapositive (if no balloons, then not Jenny’s birthday party). But it misapplies it: just because it isn’t her birthday party yet doesn’t mean it’s not her birthday. By contrast, the stimulus mistakenly assumes a conditional statement is still valid when negated.
c
The new regulations ████ ██ ██████████ ████ ██ ████ ██ ███ ████████ ██████ ██ █████ █████ ████ ██ ███ ████████ ████ ██████ ██ █████ ████████████ ███ ███ ███████████ ████ ██ ███████████
This is the wrong flaw. Like (A), (C) takes a conditional statement and erroneously assumes that it’s still valid when reversed. By contrast, the stimulus takes a conditional statement and mistakenly assumes that it’s still valid when negated.
d
In the event ████ ██ ██████ ███ ████ █████ █ █████ ████ ██████ ███ █████████ ████████ ████████████ ██ ██████ ██ ██ █████ ██████████ █ ████ ████ ██ ██ ███ ████████
The author identifies a conditional relationship (if late flight, then miss meeting). He negates the sufficient condition (flight on time), and assumes the necessary condition is also negated (meeting not missed).
This is the same flaw we saw in the stimulus: negating a sufficient condition does not automatically justify negating a necessary condition.
e
When the law ██ █████████ ████ ██████ ███ ███████ ███ ██ ███ ██ ██ █████ ██ ███████ ███ ███ ██ ███ █████████
This is a valid argument, unlike the stimulus. The conclusion of (E) is the contrapositive of the initial statement: the conditions are switched and negated. This is valid, as opposed to merely switching or merely negating the conditions. (Merely negating them is the flaw in the stimulus and (D), the correct answer choice.)