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When a person uses a false dichotomy in their argument, they believe that there are only two possible options. This means that they present a situation as if there are only two choices available, when in reality, there may be more options to consider. This can be a misleading tactic used to oversimplify a complex issue or limit the range of choices in a discussion.
For example, if someone argues that you're either with them or against them on a particular issue, without acknowledging that there could be a middle ground or other perspectives to consider, they are using a false dichotomy.
In summary, believing in a false dichotomy in an argument indicates a black-and-white thinking approach that disregards the possibility of multiple options or nuances in a situation.