This thread shall - without being loaded down already in subject and introduction by White Wolf mess ups - gather sources and works about the ideas people in the middle-ages had about their world.
Of course it would be far too easy to just list the great authors and authorities (all of Boethius, all of Augustinus, all of Hieronymus, all of Thomas Aquinas or such): anybody can figure these out for himself.
So we should restrict ourselves to ideas which might be of specific importance to Ars Magica campaigns, or which stand out as original. And the more accessible the sources or works are, the better.
Here's a few, meant to show the diversity of such peoples' ideas:
(1) Jeremiah Genest's 'De Mirabilibus Mundi':
(2) The wiki-entry to Roger von Helmarshausen, who wrote under the nom de plume Theophilus in ca. 1125 AD the 'De diversibus artibus', a treatise on the arts and crafts of his time:
Some of the Latin text of 'De diversibus artibus' is on the internet, under:
fh-augsburg.de/~harsch/Chron ... _da00.html
Editions and translations are listed here:
fh-augsburg.de/~harsch/Chron ... the_f.html
I didn't find any translations freely accessible on the internet.
(3) Here's links to pages about an interesting 13th century heresy originating from the University of Paris, the Amalricians:
newadvent.org/cathen/01379b.htm - old and very catholic, but also very precise about church teachings
(Note that the wiki-entry about Amalricians - en.wikipedia.org/wiki/Amalrician - first borrows the catholic encyclopedia entry above, and then branches into dubious speculation about the sect's further existence, mixing it up with other heresies.)
A good new account on Amalric, unfortunately in German and not on the internet, is in:
Heinrich Fichtenau: Ketzer und Professoren, C.H.Beck MÃ¼nchen 1992, ISBN 3-406-36458-6, p.280-284 and passim (see the book's register).