Ars Magus (術式 Jutsushiki, Technique Formula) is the use of science to manipulate seithr. The knowledge for how to perform each ars is recorded in a Grimoire.
During the Dark War, Konoe A. Mercury of the Six Heroes passed her Magic to mankind but humans were unable to use Magic in its basest form as they were not capable of handling or manipulating it. Therefore, in order to make use of it, humanity did a great deal of research into the subject and eventually developed what is now known as "Ars Magus". By using science to manipulate seithr that came from the Black Beast, ars magus could be used to many ends. In order to use ars magus, one would have to recite the ars, or "spell". When ars magus was first developed, the creators found that the power and effectiveness of a grimoire varied with its user. Since that discovery, mankind was able to produce more efficient ars magus. Unfortunately, Ars Magus alone was not enough to defeat the Black Beast and mankind knew they would need more power and developed Ars Armagus, or "Armagus".
It was completed approximately within half a year, thanks to Yūki Terumi's massive knowledge of seithr and the Black Beast. As normal people do not posses the ability to manipulate seithr, a Grimoire, a type of converter, changes the gathered seithr within the ars magus. Indifference to Magic, it won't work without seithr.
Ars Magus Aptitude test
The Ars Magus aptitude test (術式適正 Jutsushiki tekisei) is a simple test that is based on Ars Magus synchronicity. The test itself consist of activating a base Ars Magus. The aptitude is measured by the amount of seithr that the tester requires to activate it. An average person requires 20 units of seithr for the activation, while someone like Tao needs only 10.