 While it is possible to distinguish these different roles the “if” clause may play (there may be others too), it is not always easy to isolate them in every case. The appeal to “reasons why” and “reasons for thinking” enables us to identify what seem to be ambiguities both in the word “if” and in the terminology of necessary and sufficient conditions. Unfortunately, the concept of what is explanatory itself may be too vague to be very helpful here, for we can explain a phenomenon by citing a reason for thinking it is the case, or by citing a reason why it is the case. A similar vagueness infests the word “because”, as we see below. Consider, for example, cases where mathematical, physical or other laws are involved (one locus classicus for this issue is Sellars 1948). The truth of “that figure is a polygon” is sufficient for inferring “the sum of that figure’s exterior angles is 360 degrees”. Likewise, from “the sum of the figure’s exterior angles is not 360 degrees” we can infer “the figure is not a polygon”. Such inferences are not trivial. Rather they depend on geometrical definitions and mathematical principles, and so this is a case of mathematically necessary and sufficient conditions. But it appears quite plausible that mathematical results also give us at least a reason for thinking that because a figure is a polygon its exterior angles will sum to 360 degrees. We may even be able to think of contexts in which someone claims that a figure’s being a polygon is a reason why its exterior angles sum to 360 degrees. And it might not be unnatural for someone to remark that a certain figure is a polygon because its exterior angles sum to 360 degrees.