Asking the Right Questions, A Guide to Critical Thinking, 8th Ed

Are There Any Fallacies in the Reasoning? 89
REASONS:
1. We approve drugs when a consensus of their medical value has been
reached, and a recent survey shows a consensus approving marijuana as
a medical treatment.
2. A California association supports medical marijuana use.
First, we should note that none of the reasons points out a specific advantage of
medical marijuana; thus we should be wary from the start. Next, a close look at
the wording in the first reason shows a shift in meaning of a key term, and this
shift tricks us. The meaning of the word consensus shifts in such a way that it looks
like she has made a relevant argument when she has not. Consensus for drug
approval usually means the consensus of scientific researchers about its merits,
which is a very different consensus than the agreement of the American public
on an opinion poll. Thus the reason fails to make sense, and we should reject it.
We call this mistake in reasoning the equivocation fallacy. Whenever you see a key
word in an argument used more than once, check to see that the meaning has
not changed; if it has, be alert to the equivocation fallacy. Highly ambiguous
terms or phrases are especially good candidates for the equivocation fallacy.
F: Equivocation: A key word is used with two or more meanings in an argument
such that the argument fails to make sense once the shifts in meaning are recognized.
Well, even if there is tricky use of the word "consensus," don't the survey
results by themselves still support the conclusion? They do only if we accept the
assumption that when something is popular, then it must be good—a mistaken
assumption. The public often has not sufficiently studied a problem to provide a
reasoned judgment. Be wary of appeals to common opinion or to popular sentiment.
We label this mistake in reasoning the appeal to popularity fallacy.
F: Appeal to Popularity (Ad populum): An attempt to justify a claim by appealing to
sentiments that large groups of people have in common; falsely assumes that anything
favored by a large group is desirable.
Now, carefully examine the author's second reason. What assumption is
being made? To prove that medical marijuana is desirable, she appeals to questionable
authorities—a California Association. A position is not good just because
the authorities are for it. What is important in determining the relevance of
such reasoning is the evidence that the authorities are using in making their
judgment. Unless we know that these authorities have special knowledge about
this issue, we must treat this reason as a fallacy. Such a fallacy is called the
Appeal to Questionable Authority fallacy.