Jones's censure was echoed in England by Sir John Chardin and Richardson, in Germany by Meiners. Richardson tried to give a scientific character to the attacks of Jones by founding them on philological grounds. That the "Avesta" was a fabrication of modern times was shown, he argued, by the number of Arabic words he fancied he found both in the Zend and Pahlavi dialects, as no Arabic element was introduced into the Persian idioms earlier than the seventh century; also by the harsh texture of the Zend, contrasted with the rare euphony of the Persian; and, lastly, by the radical difference between the Zend and Persian, both in words and grammar. To these objections, drawn from the form, he added another derived from the uncommon stupidity of the matter.

In Germany, Meiners, to the charges brought against the newly-found books, added another of a new and unexpected kind, namely, that they spoke of ideas unheard of before, and made known new things. "Pray, who would dare ascribe to Zoroaster books in which are found numberless names of trees, animals, men, and demons, unknown to the ancient Persians; in which are invoked an incredible number of pure animals and other things, which, as appears from the silence of ancient writers, were never known, or at least never worshipped, in Persia? What Greek ever spoke of Hôm, of Jemshîd, and of such other personages as the fabricators of that rhapsody exalt with every kind of praise, as divine heroes?"

Anquetil and the "Avesta" found an eager champion in the person of Kleuker, professor in the University of Riga. As soon as the French version of the "Avesta" appeared, he published a German translation of it, and also of Anquetil's historical dissertations. Then, in a series of dissertations of his own, he vindicated the authenticity of the Zend books. Anquetil had already tried to show, in a memoir on Plutarch, that the data of the "Avesta" fully agree with the account of the Magian religion given in the treatise on "Isis and Osiris." Kleuker enlarged the circle of comparison to the whole of ancient literature.

In the field of philology, he showed, as Anquetil had already done, that Zend has no Arabic elements in it, and that Pahlavi itself, which is more modern than Zend, does not contain any Arabic, but only Semitic words of the Aramean dialect, which are easily accounted for by the close relations of Persia with Aramean lands in the time of the Sassanian kings. He showed, lastly, that Arabic words appear only in the very books which Parsi tradition itself considers modern.

Another stanch upholder of the "Avesta" was the numismatologist Tychsen, who, having begun to read the book with a prejudice against its authenticity, quitted it with a conviction to the contrary. "There is nothing in it," he writes, "but what befits remote ages, and a man philosophizing in the infancy of the world. Such traces of a recent period as they fancy to have found in it, are either due to misunderstandings, or belong to its later portions. On the whole there is a marvellous accordance between the 'Zend-Avesta' and the accounts of the ancients with regard to the doctrine and institutions of Zoroaster. Plutarch agrees so well with the Zend books that I think no one will deny the close resemblance of doctrines and identity of origin. Add to all this the incontrovertible argument to be drawn from the language, the antiquity of which is established by the fact that it was necessary to translate a part of the Zend books into Pahlavi, a language which was growing obsolete as early as the time of the Sassanides. Lastly, it cannot be denied that Zoroaster left books which were, through centuries, the groundwork of the Magic religion, and which were preserved by the Magi, as shown by a series of documents from the time of Hermippus. Therefore I am unable to see why we should not trust the Magi of our days when they ascribe to Zoroaster those traditional books of their ancestors, in which nothing is found to indicate fraud or a modern hand."

Two years afterwards, in 1793, was published in Paris a book which, without directly dealing with the "Avesta," was the first step taken to make its authenticity incontrovertible. It was the masterly memoir by Sylvestre de Sacy, in which the Pahlavi inscriptions of the first Sassanides were deciphered for the first time and in a decisive manner. De Sacy, in his researches, had chiefly relied on the Pahlavi lexicon published by Anquetil, whose work vindicated itself thus—better than by heaping up arguments—by promoting discoveries. The Pahlavi inscriptions gave the key, as is well-known, to the Persian cuneiform inscriptions, which were in return to put beyond all doubt the genuineness of the Zend language.

Tychsen, in an appendix to his Commentaries, pointed to the importance of the new discovery: "This," he writes, "is a proof that the Pahlavi was used during the reign of the Sassanides, for it was from them that these inscriptions emanated, as it was by them—nay, by the first of them, Ardeshîr Bâbagân—that the doctrine of Zoroaster was revived. One can now understand why the Zend books were translated into Pahlavi. Here, too, everything agrees, and speaks loudly for their antiquity and genuineness."

About the same time Sir William Jones, then president of the Royal Asiatic Society, which he had just founded, resumed in a discourse delivered before that society the same question he had solved in such an off-hand manner twenty years before. He was no longer the man to say, "Sied-il à un homme né dans ce siècle de s'infatuer de fables indiennes?" and although he had still a spite against Anquetil, he spoke of him with more reserve than in 1771. However, his judgment on the "Avesta" itself was not altered on the whole, although, as he himself declared, he had not thought it necessary to study the text. But a glance at the Zend glossary published by Anquetil suggested to him a remark which makes Sir William Jones, in spite of himself, the creator of the comparative grammar of Sanscrit and Zend. "When I perused the Zend glossary," he writes, "I was inexpressibly surprised to find that six or seven words in ten are pure Sanscrit, and even some of their inflexions formed by the rules of the Vyácaran, as yushmácam, the genitive plural of yushmad. Now M. Anquetil most certainly, and the Persian compiler most probably, had no knowledge of Sanscrit, and could not, therefore, have invented a list of Sanscrit words; it is, therefore, an authentic list of Zend words, which has been preserved in books or by tradition; it follows that the language of the Zend was at least a dialect of the Sanscrit, approaching perhaps as nearly to it as the Prácrit, or other popular idioms, which we know to have been spoken in India two thousand years ago." This conclusion, that Zend is a Sanscrit dialect, was incorrect, the connection assumed being too close; but it was a great thing that the near relationship of the two languages should have been brought to light.

In 1798 Father Paulo de St. Barthélemy further developed Jones's remark in an essay on the antiquity of the Zend language. He showed its affinity with the Sanscrit by a list of such Zend and Sanscrit words as were least likely to have been borrowed, viz., those that designate the degrees of relationship, the limbs of the body, and the most general and essential ideas. Another list, intended to show, on a special topic, how closely connected the two languages are, contains eighteen words taken from the liturgic language used in India and Persia. This list was not very happily drawn up, as out of the eighteen instances there is not a single one that stands inquiry; yet it was a happy idea, and one which has not even yet yielded all that it promised. His conclusions were that in a far remote antiquity Sanscrit was spoken in Persia and Media, that it gave birth to the Zend language, and that the "Zend-Avesta" is authentic: "Were it but a recent compilation," he writes, "as Jones asserts, how is it that the oldest rites of the Parsis, that the old inscriptions of the Persians, the accounts of the Zoroastrian religion by the classical writers, the liturgic prayers of the Parsis, and, lastly, even their books do not reveal the pure Sanscrit, as written in the land wherein the Parsis live, but a mixed language, which is as different from the other dialects of India as French is from Italian?" This amounted, in fact, to saying that the Zend is not derived from the Sanscrit, but that both are derived from another and older language. The Carmelite had a dim notion of that truth, but, as he failed to express it distinctly, it was lost for years, and had to be rediscovered.

The first twenty-five years of this century were void of results, but the old and sterile discussions as to the authenticity of the texts continued in England. In 1808 John Leyden regarded Zend as a Prácrit dialect, parallel to Pali; Pali being identical with the Magadhi dialect and Zend with the Sauraseni. In the eyes of Erskine, Zend was a Sanscrit dialect, imported from India by the founders of Mazdeism, but never spoken in Persia. His main argument was that Zend is not mentioned among the seven dialects which were current in ancient Persia according to the Farhang-i Jehangiri, and that Pahlavi and Persian exhibit no close relationship with Zend.

In Germany, Meiners had found no followers. The theologians appealed to the "Avesta," in their polemics, and Rhode sketched the religious history of Persia after the translations of Anquetil.

Erskine's essay provoked a decisive answer from Emmanuel Rask, one of the most gifted minds in the new school of philology, who had the honor of being a precursor of both Grimm and Burnouf. He showed that the list of the Jehangiri referred to an epoch later than that to which Zend must have belonged, and to parts of Persia different from those where it must have been spoken; he showed further that modern Persian is not derived from Zend, but from a dialect closely connected with it; and, lastly, he showed what was still more important, that Zend was not derived from Sanscrit. As to the system of its sounds, Zend approaches Persian rather than Sanscrit; and as to its grammatical forms, if they often remind one of Sanscrit, they also often remind one of Greek and Latin, and frequently have a special character of their own. Rask also gave the paradigm of three Zend nouns, belonging to different declensions, as well as the right pronunciation of the Zend letters, several of which had been incorrectly given by Anquetil. This was the first essay on Zend grammar, and it was a masterly one.

The essay published in 1831 by Peter von Bohlen on the origin of the Zend language threw the matter forty years back. According to him, Zend is a Prácrit dialect, as it had been pronounced by Jones, Leyden, and Erskine. His mistake consisted in taking Anquetil's transcriptions of the words, which are often so incorrect as to make them look like corrupted forms when compared with Sanscrit. And, what was worse, he took the proper names in their modern Parsi forms, which often led him to comparisons that would have appalled Ménage. Thus Ahriman became a Sanscrit word ariman, which would have meant "the fiend"; yet Bohlen might have seen in Anquetil's work itself that Ahriman is nothing but the modern form of Angra Mainyu, words which hardly remind one of the Sanscrit ariman. Again, the angel Vohu-manô, or "good thought," was reduced, by means of the Parsi form Bahman, to the Sanscrit bâhumân, "a long-armed god."

At length came Burnouf. From the time when Anquetil had published his translation, that is to say during seventy years, no real progress had been made in knowledge of the Avesta texts. The notion that Zend and Sanscrit are two kindred languages was the only new idea that had been acquired, but no practical advantage for the interpretation of the texts had resulted from it. Anquetil's translation was still the only guide, and as the doubts about the authenticity of the texts grew fainter, the authority of the translation became greater, the trust reposed in the "Avesta" being reflected on to the work of its interpreter. The Parsis had been the teachers of Anquetil; and who could ever understand the holy writ of the Parsis better than the Parsis themselves? There was no one who even tried to read the texts by the light of Anquetil's translation, to obtain a direct understanding of them.

About 1825 Eugène Burnouf was engaged in a course of researches on the geographical extent of the Aryan languages in India. After he had defined the limits which divide the races speaking Aryan languages from the native non-brahmanical tribes in the south, he wanted to know if a similar boundary had ever existed in the northwest; and if it is outside of India that the origin of the Indian languages and civilization is to be sought for. He was thus led to study the languages of Persia, and, first of all, the oldest of them, the Zend. But as he tried to read the texts by help of Anquetil's translation, he was surprised to find that this was not the clue he had expected. He saw that two causes had misled Anquetil: on the one hand, his teachers, the Parsi dasturs, either knew little themselves or taught him imperfectly, not only the Zend, but even the Pahlavi intended to explain the meaning of the Zend; so that the tradition on which his work rested, being incorrect in itself, corrupted it from the very beginning; on the other hand, as Sanscrit was unknown to him and comparative grammar did not as yet exist, he could not supply the defects of tradition by their aid. Burnouf, laying aside tradition as found in Anquetil's translation, consulted it as found in a much older and purer form, in a Sanscrit translation of the Yasna made in the fifteenth century by the Parsi Neriosengh in accordance with the old Pahlavi version. The information given by Neriosengh he tested, and either confirmed or corrected, by a comparison of parallel passages and by the help of comparative grammar, which had just been founded by Bopp, and applied by him successfully to the explanation of Zend forms. Thus he succeeded in tracing the general outlines of the Zend lexicon and in fixing its grammatical forms, and founded the only correct method of interpreting the "Avesta." He also gave the first notions of a comparative mythology of the "Avesta" and the "Veda," by showing the identity of the "Vedic Yama" with the "Avesta Yima," and of Traitâna with Thraêtaona and Ferìdûn. Thus he made his "Commentaire sur le Yasna" a marvellous and unparalleled model of critical insight and steady good sense, equally opposed to the narrowness of mind which clings to matters of fact without rising to their cause and connecting them with the series of associated phenomena, and to the wild and uncontrolled spirit of comparison, which, by comparing everything, confounds everything. Never sacrificing either tradition to comparison or comparison to tradition he knew how to pass from the one to the other, and was so enabled both to discover facts and to explain them.

At the same time the ancient Persian inscriptions at Persepolis and Behistun were deciphered by Burnouf in Paris, by Lassen in Bonn, and by Sir Henry Rawlinson in Persia. Thus was revealed the existence, at the time of the first Achaemenian kings, of a language closely connected with that of the "Avesta," and the last doubts as to the authenticity of the Zend books were at length removed. It would have required more than an ordinary amount of scepticism to look still upon the Zend as an artificial language, of foreign importation, without root in the land where it was written, and in the conscience of the people for whom it was written, at the moment when a twin language, bearing a striking likeness to it in nearly every feature, was suddenly making itself heard from the mouth of Darius, and speaking from the very tomb of the first Achaemenian king. That unexpected voice silenced all controversies, and the last echoes of the loud discussion which had been opened in 1771 died away unheeded.