A form of syllogism in scholastic logic. The name is from a medieval mnemonic poem, where its vowels reflect the syllogism's standard form of EAO-4.
Although it works fine in Aristotelian logic, modern logic considers Fesapo invalid because it commits what is called the existential fallacy, where we presuppose that a set has members. Fesapo's conclusion, a particular negative, is valid only if the classes of the universal premises have members, but it is not necessarily the case that they do. A third premise – that some M exist – is required to make the syllogism a valid rule.
Other syllogisms guilty of the existential fallacy are Bramantip, Darapti and Felapton.
Barbara, Celarent, Darii, Ferio que prioris;
Cesare, Camestres, Festino, Baroko secundae;
Tertia, Darapti, Disamis, Datisi, Felapton,
Bokardo, Ferison, habet; Quarta in super addit
Bramantip, Camenes, Dimaris, Fesapo, Fresison