3. As things are now,10those who have composed Arts of Speechhave worked on a small part of the subject; for only pisteis11are artis-tic (other things are supplementary), and these writers say nothingabout enthymemes, which is the “body” of persuasion,12while theygive most of their attention to matters external to the subject
Enthymemes are concluding a premise where one is left out or assumed to be known. This is the majority of all persuasion and Heinrich touches on this as well. It is a syllogism with the obvious logical bridge left out.