Having a bad day, Stoppard's character Guildernstern says Probability is a factor which operates within natural forces. Probability is not operating as a factor. Inclusion suggests that all the group characteristics are shared by group members.
Exclusion suggests that no common characteristics are shared.
Overlap suggests that some characteristics of one group may match some characteristics of another, while other characteristics are not shared. Insects, having no bones, are not vertebrates by exclusion, since they do not share the defining characteristic. An argument can be deemed sound if its structure is valid and its premises are true or at least acceptable . The premises are both presumably true, the structure is valid, and the conclusion is necessary, so the argument is sound. As in this case, the premises may be based on experience rather than proven fact, and are more likely to be acceptable than true .