As a Hebrew scholar Cappel made a special study of the history of the Hebrew
Masoretic Text of the
Bible, which led him to the conclusion that the
vowel points and accents are not an original part of the Hebrew language, but had been inserted by the
Masorete Jews, no earlier than the 5th century; he also concluded that the primitive Hebrew characters are those now known as the Samaritan, while the square characters are
Aramaic and were substituted for the more ancient at the time of the
Babylonian captivity. Cappel's views were not a complete novelty. Nearly a century earlier,
Elias Levita (1469–1549) demonstrated in 1538 that neither
Jerome nor the
Talmud showed any acquaintance with the vowel points, a comparatively recent Jewish invention. In response to the claim that Protestants, in spite of their claim to follow nothing but Scripture alone (
sola scriptura), were thus dependent in reality on Jewish tradition, many Protestants declared that the vowel points were in fact ancient and an essential part of the divinely inspired Scripture. Foremost among the upholders of this view were
Johannes Buxtorf senior and his son
Johannes Buxtorf II. In 1634 Cappel had already completed work on a second important work,
Critica sacra: sive de variis quae in sacris Veteris Testamenti libris occurrunt lectionibus (Sacred Criticism: Variant Readings in the Books of the Old Testament), but because of the fierce opposition of his co-religionists was able to print it only in 1650, by aid of a son, who had turned Catholic (according to the
Encyclopædia Britannica Eleventh Edition) or (according to Michael C. Legaspi in 2010) of the Catholic priest-scholar
Jean Morin. In this book, Cappel not only raised questions about the age of the vowel points in the Hebrew Bible: he denied that even the surviving consonantal Hebrew text preserved the autographs of scripture. He distinguished between the divinely inspired content of Scripture and the wholly human process of its transmission in texts that are produced by human hands with variants due above all to scribal errors and that need emendation with the help of the versions and of conjecture. The variant readings in the text and the differences between the ancient versions and the Masoretic text convinced him that the idea of the integrity of the Hebrew text, as commonly held by Protestants, was untenable. This amounted to an attack on the verbal inspiration of Scripture. Bitter, however, as was the opposition to his views, it was not long before his results were accepted by scholars. == Other writings ==