Fallacies are flawed arguments that can seem good even to capable reasoners. They come in two types: formal fallacies, which involve structural errors, and informal fallacies, which rely on weak or irrelevant evidence. Learning to spot fallacies is important. When left unchecked, not only do fallacies undermine our own reasoning, but they can also derail cooperative endeavors, such as scientific research, technological innovation, or any other activity whose success requires that we reason well together.