Combining Multiple Arguments for the Same Proposition

§5.3

When you have multiple arguments for the same proposition, first assess each individually. If any of them taken by itself establishes the proposition as certain, then the proposition is certain, regardless of the quality of the others.

In some cases, when multiple arguments each independently establish a conclusion as possible or probable, their force can add up, making the conclusion more and more probable, and ultimately certain. This is pattern of reasoning is common in induction and especially in inference to the best explanation. However, in order for the force of arguments to add up in this manner, they cannot remain wholly separate arguments, each offered in isolation from the others. They need to be brought together in what we might call a “master argument,” to show how each of the arguments reinforces the others. For example, when a certain hypothesis turns out to be the best explanation not just of one effect, but of many very different ones, and when it can be shown that the other possible explanations of each effect cannot explain the others, this strengthens our reason for believing that the hypothesis is the correct explanation for each of the effects, and so several inferences to the best explanation are integrated together into one overarching and increasingly powerful argument for the hypothesis. This is how the theory that matter is made of atoms was proven over the course of the 19th Century, as it became apparent that it could explain a wider and wider range of phenomena.