Logic / Unit 1 Logic 026 worked example, 60 to 90 seconds
Diagramming an Argument
The idea
Diagramming an argument means numbering every statement in a passage, then drawing the support relations between them as arrows, so the structure stands on its own apart from the prose.
In practice
Take this passage. One: the new bus route should be approved. Two: it cuts commute times for the east side. Three: the city's own traffic study projects a nine-minute saving. Four: the route pays for itself within three years. Number each statement, then mark the indicator words. Statement one is the final conclusion — everything else is offered in its support. Statement three backs statement two, so those chain: three to two. Statements two and four each support statement one on their own, so each gets its own arrow converging on one. Read the diagram back as a check: the traffic study backs the commute claim, and the commute claim and the cost claim independently back the recommendation.
Where people slip
Convergent and linked support look alike on the page and behave differently under attack. Premises that each support the conclusion alone get separate arrows; premises that work only in combination get bracketed together into one. Drawing linked premises as convergent makes an argument look sturdier than it is, because losing one leg then appears survivable when it is not.
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