What is the “Thema” in Thema Mundi?

The popularization of Hellenistic astrology over the past few decades has introduced the term “Thema Mundi” into modern astrological discourse. Most authors on the topic explain that “thema” means “theme” and is a synonym for “sky map” or “horoscope chart” in the literature. For example, the wikipedia entry provides a diagram of the Thema Mundi and states (italics mine):

“The Thema Mundi (‘World Theme’, with ‘theme’ being a word that also means chart) was a mythical horoscope used in Hellenistic astrology that shows the supposed positions of the seven visible planets (including the Sun and Moon) at the beginning of the universe. It purports to exemplify the logic behind the sign rulerships, exaltations, and meanings of the aspects, among other things. The purely symbolic nature of the chart is readily perceived from the impossible positions of Venus and Mercury in it.”

Image of Thema Mundi from wikipedia

The word “thema” comes into English from the Latin thema, which in turn comes from the ancient Greek théma (θέμα). Over the centuries thema has been used to mean or refer to:

1) a subject, theme, main topic, issue, thesis, subject of discourse.
2) a musical theme or melodic subject of a musical composition. 
3) an administrative military division of the Byzantine Empire (330 CE = 1453 CE).
4) in Stoic philosophy, the principles used to derive a set of statements about a topic from other or prior arguments, in other words, the principles that underlie the reasoning used to justify that one’s view of a matter is correct.

Wikipedia states that the Thema Mundi “purports to exemplify the logic behind the sign rulerships, exaltations, and meanings of the aspects, among other things.” In this sense, the “Thema” of Thema Mundi means the basic principles or assumptions that underlie Hellenistic astrology and from which other statements about this species of astrology can be logically derived. In fact, the Thema Mundi is a hypothetical sky map of the beginning of the world which serves as a visual representation of the fundamental underlying assumptions (presuppositions) of Hellenistic astrology from which the multitude of its other principles, philosophical beliefs, definitions, etc., can be derived.

One can see how well the analogy to a Byzantine administrative military division fits the concept of a Thema Mundi. Both serve to organize and establish first principles for their subject matter so that, in the case of the military for example, the army can function as a cohesive interconnected unit to accomplish its mission.