Semitic languages
The similarity of the Hebrew, Arabic and Aramaic languages was accepted by Jewish and Islamic scholars since medieval times. The languages were familiar to Western European scholars due to historical contact with neighbouring Near Eastern countries and through Biblical studies, and a comparative analysis of Hebrew, Arabic, and Aramaic was published in Latin in 1538 by Guillaume Postel. Almost two centuries later, Hiob Ludolf described the similarities between these three languages and the Ethiopian Semitic languages. However, neither scholar named this grouping as "Semitic".
Arabic definite article
While the Proto-Semitic language did not have any articles, the most likely theory is that the article "al"- comes from the same proto-Semitic source as the Hebrew definite article "ha-". That theory is based primarily on the fact that the two share many similarities. Both particles are prefixed to nouns, and both geminate with certain following letters. Moreover, neither particle is prefixed to non-final nouns in a genitival construction. Finally, both are prefixed to relative clauses. According to David Testen, many northern and southwestern Semitic languages have particles that bear similarities to "al"-. With this fact, he posits that "al"- has a proto-Semitic antecedent.