Card 20 of 24 · Presumption
Petitio principii
Using your conclusion as your evidence.
The argument loops in a circle.
Example
“The policy is effective because it’s a good policy.”
“Why is it good?”
“Because it’s effective.”
“Assuming the starting point.” When the loop is explicit, it’s circulus in probando — “circle in proving.”
Diagnosis: The argument is technically valid (C implies C), but it can’t persuade anyone who doesn’t already accept C. The conclusion is hidden in the premises.
“The Bible is God’s word because it says so, and we trust what it says because it’s God’s word.” Presuppositional apologists argue all ultimate authorities are in some sense circular — a serious claim that should be defended openly, not left implicit.
“Miracles can’t happen, so the resurrection accounts must be legendary.” Hume-style arguments risk assuming naturalism to prove naturalism.
“Abortion is murder because it’s the wrongful killing of a person.” Whether the fetus is a person is the contested premise and needs its own argument — on either side.
Trail recovery
Find independent support. Ask what reason someone who doesn’t already agree could accept. Charitably, the speaker may have good reasons they haven’t yet articulated.
Try saying: “What would convince someone who doesn’t already share that starting point?”
Keep questioning. Keep exploring.