Deriving “Some animals are humans” from “All humans are animals” assumes the relevant human class has members. The textbook develops the traditional rule through classes that do have instances.
A universal affirmative includes every subject-instance under its predicate. A particular affirmative claims at least one shared instance.
Modern formal systems can interpret universal statements over empty classes differently. Here we retain the classical rule taught in *Al-Mirqat*, while identifying the existential assumption rather than concealing it.