Heidegger's Bible Handbook: Harmony of the Gospels: The Legitimacy of Gospel Harmonies
4. The Harmony of the Gospels is defined. The committing of it to writing is not illegitimate, since the Evangelists themselves furnish notices of order; but useful in the highest degree: hence also attempted by ancient and more recent men.
Therefore, in this way an Evangelical Harmony, like a certain Diatessaron musical consonance, is evident, in which the history of the life, death, resurrection, and ascension of Christ from the four Evangelists compared together, both things common, or related by all, and some things singular, but tied to the common things, are represented in such a way that the order of the times, elicited from certain indicators, is everywhere kept, and those individual things are exhibited, arranged in the sequence of time, in which they happened. The writing of this harmony, although not composed by the Evangelists themselves, since it was not their purpose to write Annals or Day-books, or a ἡμερολογίαν/Calendar; is not rash, or contrary to the intention of the Evangelists, since they themselves, Luke especially, by a noting of the circumstances hint that the order both of the times and of the matters conducted in the Gospel history is able to be investigated and discovered in most narrations; but in the rest plausible reasons and conjectures not absurd should suffice for religious piety, which seeks, not precision of moments, but some order ἐν πλάτει, in general; neither ought it to be judged useless. For such a harmony both makes for the blunting the calumnies of the impious, Porphyry, Celsus, and the like, charging that the history of the Gospel is not consistent with itself, and coheres poorly; and for the confirmation of the pious, who, although believing savingly, are often disturbed by a certain appearance or opinion of discrepancy or confusion in the reading of the Gospel history: and it most beautifully illustrates many things in the Gospel history, so that it is able to discharge the function of a commentary not to be despised. But this consideration also has much religious charm. For, if in profane histories is commended the study of those that out of various writers and commentaries set the lives of excellent men in the order of times and of matters conducted, demonstrated as far as it is able to be done; for pious minds are not able not to be greatly delighted, when they are able by a certain order to embrace in memory, and to keep in mind, the whole life and the saving acts of our altogether sweet Savior. Hence, both the ancients, Tatian, Origen of Alexandria, Eusebius of Cæsarea, Theophilus of Antioch, and Augustine: and the more recent following Peter Comestor, Bonaventure, Ludolph of Saxony, and Gerson, namely, Calvin, Osiander, Bugenhagen, Chemnitz, Vossius, Richardson, Cartwright, Lightfoot, and others also of the Papists, whom we shall reckon up in their place: exercised themselves to the utmost of their power to construct a harmony.
 Porphyry (c. 232-c. 304) studied in Rome under Plotinus. He endeavored to make the obscure Neoplatonism of Plotinus intelligible to the popular reader.  Celsus was a second century Greek philosopher and opponent of Christianity. Excerpts from his The True Word are found in Origen’s Contra Celsum.  Tatian the Assyrian (c. 120-c. 180) was a Christian theologian and apologist. He is most remembered for his Diatessaron, his harmony of the four gospels, which was used in the Syriac church until the fifth century. In his Oratio ad Græcos, he extols the virtues and antiquity of Christianity, and critiques paganism. Some shadow has been cast over his name by accusations of heresy from Irenæus and Eusebius.  Quæstiones ad Stephanum et Marinum.  De consensu evangelistarum.  The Historia Scholastica (completed circa 1173) was a Biblical paraphrase, presenting a universal history in a popular manner. It was written by Petrus Comestor (died 1178), a prolific theological writer (although much of his work had gone unpublished, including commentaries on the Gospels), and chancellor of the theological school at Notre-Dame. The Historia Scholastica was part of the core curriculum of many universities, even into the fifteenth century.  Life of Our Lord and Savior Jesus Christ.  Monotesseron.  Commentary on a Harmony of the Evangelists, Matthew, Mark, and Luke.  Conciliata ex Euangelistis historia passi Christi et glorificati, cum annotationibus.  Gerardi Ioannis Vossii Harmoniæ evangelicæ de passione, morte, resurrectione, ac ascensione Iesu Christi, servatoris nostri, libri tres. Gerhard Johann Vossius (1577-1649) was a Dutch classical scholar and theologian. In 1619, his Historia Pelagiana brought him into suspicion of Arminianism.  Harmonia evangelica commentario illustrata.