An existing case can show that something is possible or that a principle has been accepted somewhere. It cannot, by itself, show that the same principle belongs in a new setting.
A successful pilot can rebut “this cannot work anywhere.” It does not yet establish that the approach will scale, transfer, or remain economical.
How it works
An existence proof answers a narrow question: does at least one relevant example exist? A working prototype can challenge a claim of technical impossibility. A prior agreement can show that a contractual arrangement has been used. Those conclusions are narrower than “this is a good idea” or “this will work for us.”
A precedent about a social rule is narrower still. The fact that an institution draws a distinction shows that the distinction has been adopted; it does not establish that the distinction is justified or transferable.
Make the comparison explicit
State precisely which claim the example challenges. Identify the feature that makes it relevant, then examine the differences: scale, incentives, constraints, cost, and the people affected.
Give the opposing argument its strongest form. If the objection is that an approach cannot scale economically, a small successful pilot does not answer it. The evidence must address the actual objection.
When it misleads
A familiar institution can lend authority to an unrelated proposal. Avoid reasoning that an existing practice must be right because it exists, or that acceptance in one setting requires acceptance in another.
Use the precedent to open the next question, not to close the debate. What made it work there, and what evidence would show that those conditions also hold here?