Unstated Assumptions: The Link Between Claim and Evidence

Why evidence sometimes proves nothing

Between a claim and the evidence for it there is always a hidden step: the assumption that this kind of evidence counts for this kind of claim. Logicians call it the warrant.

• Claim: the new bus route should be cancelled.
• Evidence: it carries eleven passengers a day.
• Assumption: a public service should be judged by ridership.

The evidence is not in dispute. The assumption is — and that is where the real argument lives. Someone who believes a bus route should be judged by whether it reaches people with no alternative will look at the same eleven passengers and reach the opposite conclusion.

Commentary is the warrant made visible

This is the most useful thing to know about the essays. Students are told "add commentary" and produce restatement. Commentary is not restatement; it is the sentence that says why this evidence supports this claim — that is, the assumption written out.

Test your own paragraph: cover the evidence and read claim then commentary. If the commentary still makes sense on its own, it is probably restatement. Real commentary refers to a specific feature of the evidence and explains what makes that feature count.

Assumptions on the multiple-choice section

Questions like "the argument in lines 20–28 depends on which of the following assumptions?" are asking for the warrant. Method: state the conclusion, state the evidence, then ask what would have to be true for that evidence to matter. The answer is usually the option that, if false, would collapse the argument — so test each option by negating it.