The first of the Romans to produce a school grammar was Remmius Palaemon, who flourished in the first half of the first century. He had many successors in the later centuries of the Roman Empire, and the literary tradition of the school grammar continued unbroken into the Middle Ages. The most influential exponent of the subject was Aelius Donatus, whose _Ars_, written in the fourth century, was used throughout the Middle Ages. The chief writers of grammatical texts in the centuries preceding Isidore were Victorinus, Donatus, Diomedes, Charisius, and Martianus Capella in the fourth; Consentius and Phocas in the fifth; and Cassiodorus in the sixth. No new contributions were being made to the science, and these writers had no other resource than to copy their predecessors, which they did in a slavish manner.[159] The verbal similarity in all of them is so strong that it is impossible to trace with certainty the immediate source of any one of the later writers. [159] The following list of passages gives some idea of the way in which grammatical works were produced in this age. Vox sive sonus est aer ictus, id est percussus, sensibilis auditu quantum in ipso est. Probi, _Instituta Artium_ in Keil, _Grammatici Latini_, vol. vi, p. 4, 13. Vox est aer ictus sensibilis auditu, quantum in ipso est. Donati, _Ars Grammatica_. _Ibid._, vol. iv, p. 367, 5. Vox est aer ictus sensibilis auditu, verbis emissa, et exacta sensus prolatio. Sergii, _Explanationum in artem Donati, Liber I._, _Ibid._, vol. iv, p. 487, 4. Vox est aer auditu percipibilis quantum in ipso est. Marius Victorinus, _Ars Grammatica_. _Ibid._, vol. vi, p. 4, 13. Vox quid est? Aer ictus sensibilisque auditu quantum in ipso est. Maximus Victorinus, _Ars Grammatica_. _Ibid._, vol. vi, p. 189, 8. Vox articulata est aer percussus sensibilis auditu quantum in ipso est. Cassiodorus, _Institutio de Arte Grammatica_. _Ibid._, vol. vii, p. 215, 4. Vox est aer ictus sensibilis auditu, quantum in ipso est. Isidore, _Etymologiae_, 1, 15. These grammars are almost altogether made up of definitions which had become stereotyped. Isidore’s account of grammar is of somewhat more than the average length[160] found in these text-books, but its lack of solid substance, in which it differs from the books of the fourth century, measures the decline in intellectual grasp and thoroughness of the two intervening centuries. Donatus, Servius, and even Capella, stick closely to the technique of the subject and are thorough-going; their books are calculated to afford a severe discipline to the student. But in Isidore a feebleness in handling the subject is evident; he is apparently unaware of the superior importance of such subjects as conjugation and declension, and he is very easily led into confusion by the trains of thought suggested by his frequent derivations.[161]