that unlike most satem languages it has preserved the distinction of /kʷ/ and /gʷ/ from /k/ and /g/ before front vowels (merged in satem languages), and there is a debate whether Illyrian was centum or satem. On the other hand, Dacian and Thracian seem to belong to satem. The debate is often politically charged, and to be conclusive, more evidence is needed. Such evidence unfortunately may not be easily forthcoming because of a lack of sources. Illyrian The very first recorded mention of a connection between Illyrians and Albanians is in 1709, attributed to the German philosopher and mathematician Gottfried Leibniz , most famous for being the co-inventor of Calculus along with Isaac Newton. In a series of letters, he first speculated Albanian to be related to the other Slavic languages along the Adriatic, but soon changed his mind and connected the Albanian language to that of the ancient Illyrians. In terms of linguists or historians, the theory that Albanians were related to the Illyrians was proposed for the first time by the Swedish historian Johann Erich Thunmann in 1774. The scholars who advocate an Illyrian origin are numerous. Those who argue in favour of an Illyrian origin maintain that the indigenous Illyrian tribes dwelling in South Illyria (including today's Albania) went up into the mountains when Slavs occupied the lowlands, while another version of this hypothesis states that the Albanians are the descendants of Illyrian tribes located between Dalmatia and the Danube who spilled south. Some of the arguments for the Illyrian-Albanian connection have been as follows: From what is known from the old Balkan populations territories (Greeks, Illyrians, Thracians, Dacians), Albanian is spoken in a region where Illyrian was spoken in ancient times. There is no evidence of any major migration into Albanian territory since the records of Illyrian occupation. Because descent from Illyrians makes "geographical sense" and there is no linguistic or historical evidence proving a replacement, then the burden of proof lies on the side of those who would deny a connection of Albanian with Illyrian. The Albanian tribal society has preserved the ancient Illyrian social structure based on tribal units. In addition, Çabej analyzed the morphology of some tribal names and pointed out that the Illyrian suffix -at appeared in the names of Illyrian tribes, such as Docleatae, Labeatae, Autariates, Delamatae correspondends to the suffix -at appeared in the 15th century Albanian tribes names like