The climate-change debate, like all policy debates, is ultimately an argument over action. How shall we respond to the risks posed by climate change? Does the climate-change issue call for action, and if so, what type of action, and how much effort – and money – shall we expend? Listen to the debate over climate change and you will hear people making many different kinds of arguments – about whether and how the climate is changing, whether human activities are affecting the climate, how the climate might change in the future, what the effects of the changes will be and whether they matter, and the feasibility, advantages, and disadvantages of various responses. Although these arguments are distinct, when advanced in policy debate they all serve to build a case for what we should or should not do. Their goal is to convince others to support a particular course of action. This chapter lays the foundation for understanding these arguments. Section 2.1 lays out the differences between the two kinds of claims advanced in policy debates, positive and normative claims. Sections 2.2 and 2.3 then discuss how science examines and tests positive claims, and how participants in policy debates use both positive and normative claims to build arguments for – and against – proposed courses of action. Section 2.4 examines what happens when these two kinds of debates overlap, as they do whenever positive claims that scientists have examined are relevant to public action – as is clearly the case in the climate-change debate.