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To prove the common origin of languages, van Boxhorn compared etymologies, inflection patterns and grammars of Greek, Latin, Persian, Old Saxon, Dutch, German, Gothic, Russian, Danish, Swedish, Lithuanian, Czech, Croatian and Welsh. He found similarities that suggest a genetic relationship of these languages. Van Boxhorn was the first to include not only Greek, Germanic, Romance and Slavic languages in the language family, but also Persian, Sanskrit, Celtic and Baltic languages.
Marcus Zuërius van Boxhorn compared not only similar words in different languages, but whole inflection patterns and grammars. He was of the opinion that the relationship between languages must be verifiable on the basis of systematic grammatical correspondences and not only postulated on the basis of similar-looking word forms. He was the founder of the methodology for studying language that we now call the comparative method.
Van Boxhorn viewed language as an organic system and warned against loan words and Wanderworts, which spread as loanwords among languages and cultures, and which can influence the comparison of languages. A kinship of languages is often wrongly suspected on the basis of similar words, but these have been adopted from one language and originally come from another language. Van Boxhorn wanted to prevent these misinterpretations by systematically comparing inflection morphology and other grammatical features.