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, Exclusion and Overlap Syllogisms are often based on inclusion of one case within a group, exclusion of one case from a group, or overlap between a case and two or more groups.
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. Truth and Validity Formal logic is a tool for assessing argument. An argument can be deemed sound if its structure is valid and its premises are true or at least acceptable .