Logos: Evaluating Logical Appeals

Analyze how writers use evidence and reasoning, and evaluate the strength of that reasoning.

Logos is the appeal to logic: facts, statistics, examples, expert testimony, and the reasoning that connects them to a conclusion. Two kinds of reasoning are common. Deductive reasoning moves from a general principle to a specific conclusion: 'All students in the program must complete service hours; Rina is in the program; so Rina must complete service hours. ' Inductive reasoning moves from specific evidence to a general conclusion: 'In twelve schools that adopted later start times, attendance rose; later start times probably improve attendance.

' Deductive conclusions are certain if the premises are true. Inductive conclusions are only probable, and their strength depends on the quantity, quality, and representativeness of the evidence. When analyzing logos, ask whether the evidence is accurate, relevant, and sufficient, and whether the conclusion actually follows. A writer can cite many statistics and still reason badly, for instance by assuming that because two things occur together, one causes the other.

• 1Logos uses evidence and reasoning.
• 2Deductive: general to specific; certain if premises are true.
• 3Inductive: specific to general; only probable.
• 4Many statistics do not guarantee sound reasoning.