Basilides, one of the most conspicuous exponents of Gnosticism, was living at Alexandria probably as early as the first decades of the 2nd century. It is true that Eusebius, in his Chronicle, dates his first appearance from A.D. 133, but according to Eusebius, Hist. Eccl. iv. 7 §§ 6-8, Agrippa Castor, who lived under Hadrian (117-138), already wrote a polemic against him, so that his activity may perhaps be set back to a date earlier than 138. Basilides wrote an exegetical work in twenty-four books on "his" gospel, but which this was is not known. In addition to this there are certain writings by his son Isidorus περὶ προσφυοῦς ψυχῆς; Ἐξηγητικά on the prophet Parchor (παρχώρ); Ἠθικά. The surviving fragments of these works are collected and commented on in Hilgenfeld's Ketzergeschichte, 207-218. The most important fragment published by Hilgenfeld (p. 207), part of the 13th book of the Exegetica, in the Acta Archelai et Manetis c. 55, only became known in its complete form later, and was published by L. Traube in the Sitzungsbericht der Münchener Akad., phil. histor.
Kl. (1903), pp. 533-549. Irenaeus (Adv. Haer. i. 24 §§ 3-7) gives a sketch of Basilides' school of thought, perhaps derived from Justin's Syntagma. Closely related to this is the account in the Syntagma of Hippolytus, which is preserved in Epiphanius, Haer. 24, Philaster, Haer. 32, and Pseudo-Tertullian, Haer. 4. These are completed and confirmed by a number of scattered notices in the Stromateis of Clemens Alexandrinus. An essentially different account, with a pronounced monistic tendency, is presented by the so-called Philosophumena of Hippolytus (vii. 20-27; x. 14). Whether this last account, or that given by Irenaeus and in the Syntagma of Hippolytus, represents the original system of Basilides, has been the subject of a long controversy. (See Hilgenfeld p. 205, note 337.) The most recent opinion tends to decide against the Philosophumena; for, in its composition, Hippolytus appears to have used as his principal source the compendium of a Gnostic author who has introduced into most of the systems treated by him, in addition to the employment of older sources, his own opinions or those of his sect.
The Philosophumena, therefore, cannot be taken into account in describing the teaching of Basilides (see also H. Stachelin, "Die gnostischen Quellen Hippolyts" in Texte und Untersuchungen, vi. 3; and the article Gnosticism). A comparison of the surviving fragments of Basilides, moreover, with the outline of his system in Irenaeus-Hippolytus (Syntagma) shows that the account given by the Fathers of the Church is also in the highest degree untrustworthy. The principal and most characteristic points are not noticed by them. If we assume, as we must needs do, that the opinions which Basilides promulgates as the teaching of the "barbari" (Acta Archelai c. 55) were in fact his own, the fragments prove him to have been a decided dualist, and his teaching an interesting further development of oriental (Iranian) dualism. Entirely consistent with this is the information given by the Acta Archelai that Basilides, before he came to Alexandria, had appeared publicly among the Persians (fuit praedicator apud Persas); and the allusion to his having appealed to prophets with oriental names, Barkabbas and Barkoph (Agrippa in Eusebius Hist. Eccl. iv. 7 § 7). So too his son Isidorus explained the prophecies of a certain Parchor ( = Barkoph) and appealed to the prophecies of Cham (Clemens Alexandrinus, Stromat. vi. 6 § 53). Thus Basilides assumed the existence of two principles, not derivable from each other: Light and Darkness. These had existed for a long time side by side, without knowing anything of each other, but when they perceived each other, the Light had only looked and then turned away; but the Darkness, seized with desire for the Light, had made itself master, not indeed of the Light itself, but only of its reflection (species, color). Thus they had been in a position to form this world: unde nec perfectum bonum est in hoc mundo, et quod est, valde est exiguum.
This speculation is clearly a development of that which the Iranian cosmology has to tell about the battles between Ahura-Mazda and Angro-Mainyu (Ormuzd and Ahriman). The Iranian optimism has been replaced here by a strong pessimism. This material world is no longer, as in Zoroastrianism, essentially a creation of the good God, but the powers of evil have created it with the aid of some stolen portions of light. This is practically the transference of Iranian dualism to the more Greek antithesis of soul and body, spirit and matter (cf. Irenaeus i. 24 § 5: animae autem eorum solam esse salutem, corpus enim natura corruptibile existit). The fundamental dualism of Basilides is confirmed also by one or two other passages. In the parable of the rich man and Lazarus, Basilides saw the proof of naturam sine radice et sine loco rebus supervenientem (Acta Archelai). According to Clemens, Strom. iv. 12 § 83, etc., Basilides taught that even those who have not sinned in act, even Jesus himself, possess a sinful nature. It is possibly also in connexion with the dualism of his fundamental views that he taught the transmigration of souls (Origen in Ep. ad Rom. lib. v.; Opp. de la Rue iv. 549; cf.
Clemens, Excerpta ex Theodoto, § 28). Isidorus set up celibacy, though in a modified form, as the ideal of the perfect (Clemens, Strom. iii. 1 § 1, etc.) Clemens accuses Basilides of a deification of the Devil (θειάζειν τὸν διάβολον), and regards as his two dogmas that of the Devil and that of the transmigration of souls (Strom. iv. 12 § 85: cf. v. 11 § 75). It is remarkable too that Isidorus held the existence of two souls in man, a good and a bad (Clemens, Strom. ii. 20 113); with which may be compared the teaching of Mani about the two souls, which it is impossible to follow F, Ch. Baur in excluding, and also the teaching of the Pistis Sophia (translated by C. Schmidt, p. 182, etc.). According to Clemens (Strom. ii. 20 § 112), the followers of Basilides spoke of πνεύματά τινα προσηρτημένα τῇ λογικῇ φυχῇ κατά τινα τάραχον καὶ σύγχυσιν ἀρχικήν: that is to say, here also is assumed an original confusion and intermingling.