Would this injury have happened anyway if the other person had acted differently? That is the basic idea behind but-for causation. A person or business is a but-for cause of harm when the injury would not have happened but for that conduct. It is a way courts and insurers connect an act to a result. Even if someone acted carelessly, there may be no legal responsibility unless that carelessness actually helped cause the injury.

In a personal injury claim, but-for causation can decide whether compensation is available at all. It often becomes a fight over proof: medical records, witness statements, photos, timing, and expert opinions may all be used to show that the harm flowed from the incident rather than from a prior condition or some unrelated event. That can matter a lot when someone delays treatment because of cost, fear, or confusion about paperwork, since the other side may argue the injury would have happened anyway.

In New York, but-for causation often works alongside negligence, comparative fault, and proximate cause. If the claim is against a city agency or the MTA, causation still has to be proven, and there is also a strict notice of claim rule. Under New York General Municipal Law § 50-e, many claims against public entities require notice within 90 days, so waiting too long can jeopardize the case before the causation evidence is even heard.