How can I identify assumptions an online argument depends on but never states?

First, reduce the argument to its basic shape. Record the conclusion—the point the writer wants you to accept—and the reasons they actually give. Keep your wording neutral and close to the post; don’t add a reason that seems obvious to you. If the post mixes several claims, choose one conclusion to examine at a time.

For example, suppose a post says: “Our neighborhood added a bike lane, and nearby shops have more customers. The bike lane helped local businesses.” The conclusion is that the lane helped. The stated reason is that customer counts rose after it was added. The time relationship is stated; the explanation that the lane caused the increase may not be. Treat that possible causal link as something to examine, not as a fact.

Ask: “What would need to be true for these reasons to make this conclusion more likely?” Look for the bridge between them. Is the writer assuming that one event caused another, that a few examples represent a wider group, that a comparison is fair, or that no important alternative explanation applies?

For the bike-lane example, possible bridges include: the customer increase happened after the lane opened; the increase was not mainly due to another change; and the customers counted are relevant to the claim about local businesses. These are candidate assumptions, not a verdict about what happened. Phrase each as a complete sentence that could be checked or challenged. Avoid turning every imaginable possibility into an assumption: include only claims the argument needs to reach its conclusion.

Use a removal test: imagine the candidate assumption is false. Would the stated reasons still support the conclusion? If the conclusion loses an important link, the assumption is probably doing real work. If the argument still stands without it, the assumption may be optional or beside the point.

Then label each item clearly: “stated,” “unstated and needed,” or “possible but not necessary.” For instance, if another major change could explain the customer increase, the claim that the bike lane caused it is weakened. But that does not establish that the lane had no effect. The removal test identifies dependence in the reasoning; it does not prove whether an assumption is true, false, or the only explanation.

Use this short worksheet, then fill it in with the post’s own claim:

1. Conclusion: What does the writer want me to accept?
2. Stated reasons: What support do they explicitly give?
3. Missing bridge: What would have to be true for those reasons to support that conclusion?
4. Removal test: If this bridge were false, would the conclusion still follow?
5. Status: stated, unstated and needed, or possible but not necessary.
6. Next question: What information could clarify this particular assumption?

Worked example: Conclusion—“The bike lane helped local businesses.” Stated reason—“Nearby shops had more customers after it opened.” Candidate bridge—“The lane, rather than another change, contributed to the increase.” Removal test—if another change explains the increase, the evidence given does not establish the lane’s effect. Next question—“What else changed, and how were customers counted?” Keep the outcome open until you have relevant information.