In his 1647 essay, Marcus Zuerius van Boxhorn proposed the existence of a primitive common language he called "Scythian". He included in its descendants Dutch, German, Latin, Greek, and Persian, and his posthumously published "Originum Gallicarum liber" of 1654 added Slavic, Celtic and Baltic. The 1647 essay discusses, as a first, the methodological issues in assigning languages to genetic groups. For example, he observed that loanwords should be eliminated in comparative studies, and also correctly put great emphasis on common morphological systems and irregularity as indicators of relationship. A few years earlier, the Silesian physician Johann Elichmann (1601/1602–1639) already used the expression "ex eadem origine" (from a common source) in a study published posthumously in 1640. He related European languages to Indo-Iranian languages (which include Sanskrit).