It is important, for the understanding of the following Book, to determine what faculty of the mind ᾿ Επίνοια is. Eunomius, Gregory says, “makes a solemn travesty” of the word. He reduces its force to its lowest level, and makes it only “fancy the unnatural,” either contracting or extending the limits of nature, or putting heterogeneous notions together. He instances colossi, pigmies, centaurs, as the result of this mental operation. “Fancy,” or “notion,” would thus represent Eunomius' view of it. But Gregory ascribes every art and every science to the play of this faculty. “According to my account, it is the method by which we discover things that are unknown, going on to further discoveries, by means of what adjoins and follows from our first perception with regard to the thing studied.” He instances Ontology (!), Arithmetic, Geometry, on the one hand, Agriculture, Navigation, Horology, on the other, as the result of it. “Any one who should judge this faculty more precious than any other with the exercise of which we are gifted would not be far mistaken.” “Induction” might almost represent this view of it. But then Gregory does not deny that “lying wonders are also fabricated by it.” By means of it “and entertainer might amuse an audience” with fire-breathing monsters, men enfolded in the coils of serpents, etc. He calls it an inventive faculty. It must therefore be something more spontaneous than ratiocination, whether deductive or inductive; while it is more reliable than Fancy or Imagination.
This is illustrated by what S. John Damascene, in his Dialectica (c. 65), says of ᾿ Επίνοια: “It is of two sorts. The first is the faculty which analyses and elucidates the view of things undissected and in the gross (ὁ λοσχερῆ): whereby a simple phenomenon becomes complex speculatively: for instance, man becomes a compound of soul and body. The second, by a union of perception and fancy, produces fictions out of realities, i.e. divides wholes into parts, and combines those parts, selected arbitrarily, into new wholes; e.g. Centaurs, Sirens.” Analysis (scientific) would describe the one; fancy, the other. Basil and Gregory were thinking of the one, Eunomius of the other; but still both parties used the same expression.
If, then, there is one word that will cover the whole meaning, it would seem to be “Conception.” This word at all events, both in its outward form and in its intention, stands to perception in a way strictly analogous to that in which ᾿ Επίνοια stands to ῎ Εννοια. Both Conception and ᾿ Επίνοια represent some regulated operation of the mind upon data immediately given. In both cases the mind is led to contemplate in a new light its own contents, whether sensations or innate ideas. The fitness of Conception as an equivalent of ᾿ Επίνοια will be clear when we consider the real point at issue between Basil and Eunomius. Their controversy rages round the term Ungenerate. Is it, or is it not, expressive of the substance (being) of the Deity? To answer this question, it was found necessary to ascertain how such a name for the Supreme has been acquired. “By a conception,” says Basil. “No,” says Eunomius: “it would be dangerous to trust the naming of the Deity to a common operation of the mind. The faculty of Conception may and does play us false; it can create monstrosities. Besides, if the names of the Father are conceptions, the names of the Son are too; for instance, the Door, the Shepherd, the Axe, the Vine. But as our Lord Himself applied these to Himself, He would, according to you, be employing the faculty of conception; and it is blasphemous to think that He employed names which we too might have arrived at by conceiving of Him in these particular ways. Therefore, Conception is not the Source of the Divine Names; but rather they come from a perception or intention implanted in us directly from on High. Ungenerate is such a name; and it reveals to us the very substance of the Deity.” But Gregory defends Basil's position. He shows the entire relativity of our knowledge of the Deity. Ungenerate and every other name of God is due to a conception; in each case we perceive either an operation of the Deity, or an element of evil, and then we conceive of Him as operating in the one, or as free from the other; and so name Him. But there is no conception, because there is no perception, of the substance of the Deity. Scripture, which has revealed His operations, has not revealed that. “The human mind...feels after the unutterable Being in various and many-sided ways; and never chases the mystery in the light of one idea alone. Our grasping of Him would indeed be easy, if there lay before us one single assigned path to the knowledge of God; but, as it is, from the skill apparent in the Universe, we get the idea of skill in the Ruler of the Universe;...and again, when we see the execrable character of evil, we grasp His own unalterable pureness as regards this,...not that we split up the subject of such attributes along with them, but, believing that this Being, whatever it be in substance, is one, we still conceive that it has something in common with all these ideas.”
To sum up, it had suited Eunomius to try to disparage ᾿ Επίνοια so far as to make it appear morally impossible that any name of God, but especially ᾿ Αγέννητος, should be derived from such a source. He scoffs at the orthodox party for treating the privative terms for the Deity as merely privative, embodying only a “notion,” and for adhering to the truth that God's name is “above every name.” He “does not see how God can be above His works simply by virtue of such things as do not belong to Him;” this is only “giving to words the prerogative over realities.” He wants, and believes in the existence of, a word for the substance of God, and he finds it in ᾿ Αγέννητος, which according to him is not privative at all; it is the single name for the single Deity, and all the others are bound up in it. “The universal Guardian thought it right to engraft these names in our minds by a law of His creation.” “These utterances are from above.” The importance of this word to the Anomœans is obvious. Gregory, as spokesman of the Nicene party, defends the efficacy of the mental operation of conception to supply terms for the Deity, which, however, can none of them be final. God is incomprehensible. At the same time there is a spiritual insight of God (an ἔ ννοια in fact) which far surpasses Eunomius' intellectual certainty (see note p. 256).
Source: Answer to Eunomius' Second Book (New Advent)