For example, two people may argue about whether UFO's exist. If the person arguing that UFO's do exist merely means by 'UFO' flying objects which have never been identified, while the one disagreeing means by 'UFO' alien spacecraft visiting Earth, then it is quite possible for them to agree that no alien spacecraft visiting the Earth exist. But by neglecting to define their terms first, they may argue for a long time before they discover that they agree.
When questioning another's definitions and when attempting to provide
useful definitions of one's own one must know what makes for good definitions
and bad definitions. The act of pointing is ambiguous, so defining by pointing
(an ostensive definition) is not really a good way to define something.
Dictionary definitions often consist in the giving of a synonym; this could
only provide a useful definition if the synonym has just one possible meaning
and both parties know that meaning. Dictionaries do provide a useful starting
point, but the best definitions for the purposes of critical thinking are
those which give genus and differentia.
The term being defined will represent some concept.
The genus states to which more general kind of thing this concept belongs
and the differentia states how this concept differs from others (other
species) of that same general kind.
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