One of the hallmarks of van Eys's work is that, for the first time, in a truly scientific way, a large number of Basque linguistics. When, shortly after 1860, van Eys began to study the Eskuara, he found himself in the presence of an immense literature. He gave new solutions to many Basque problems, many of them definitive. Basque linguistics is indebted to van Eys for many advances, the most important of which are the establishment of a number of phonetic laws, the origin of the article determination, and a large number of reliable etymologies.