In this chapter, a new model of causation is formulated that views causal argumentation as defeasible. The new model structures many of the most common cases of causal argumentation as dialectical, meaning that the case is viewed in the context of an investigation or discussion in which two parties take part in a collaborative process of rational argumentation. The model is shown to apply very well, particularly during the initial stages of an investigation, where information is incomplete but preliminary hypotheses are formed. But it will also be argued that causal arguments need to be evaluated differently in different contexts. In scientific argumentation, there is an investigative process in which tentative hypotheses are formulated about a cause at an early stage, and then tested and refined at later stages. In legal argumentation, the method of evaluation typically is a trial or some other form of dispute resolution in which a causal claim made by one side is opposed to one made by the other side. For example, it could be a case in tort law concerning whether a toxic substance caused cancer in a population. In a criminal case, it could be a trial concerning the cause of an accident. In such cases, there are differing opposed views on what the cause of something is, and it is to such cases that the defeasible model applies best.