When we reason about the truth or falsity of a conclusion, we
rarely consider the conclusion in a vacuum. Instead, we have in
mind a range of other possible conclusions. When we think about
whether to take a certain action, we weigh it against other
actions. When we consider a hypothesis to explain why something
happened, we test it against other hypotheses. In short, thinking
often involves a choice among alternatives.
The fallacy of false alternative occurs when we fail to consider
all the relevant possibilities.
One form occurs when we consider only the extreme points of a
scale. If you told me that Diane is not rich, I would commit the
fallacy if I inferred that she is poor. Rich and poor are the
extremes on a scale that contains many intermediate degrees of
wealth.
The most subtle examples of the fallacy are those in which
relevant alternatives are excluded by some implicit, unspoken,
and thus invisible assumption.