Set-based reasoning is based around placing things into groups or categories of other similar items – sets. If an item belongs to a certain set then it must have all of the traits shared by items in that set. There are multiple possible relationships between items and sets: an item may be part of a set, such as fashion magazines are part of the set ‘books’; it may NOT be part of a set, such as fashion magazines are not part of the set ‘furniture’; or it may be a part of multiple sets, such as fashion magazines are a part of ‘books’ and ‘things men don’t get about women’.
Sets also intersect and nest within each other – two or more sets may have areas of overlap where they have items in common, or some sets may be wholly enclosed within other sets. Items part of the same set will have characteristics in common; hence a conclusion may be drawn from these relations.
It’s important not to fall into the trap of believing that if a certain item belongs to a certain set then it belongs only to that set, particularly since it is utterly impossible to list all of the sets an item belongs to. Still, this trap can be used to confound less astute opponents by listing only the sets you want to compare an item to and leaving out any others it belongs to that run counter to your point.
