The Seraphims' Teaching

Quod autem duo seraphim in Isaia leguntur, figuraliter veteris, et novi Testamenti significationem ostendunt. Quod vero faciem et pedes Dei operiunt, quia praeterita ante mundum, et futura post mundum scire non possumus, sed media tantum, eorum testimonio contemplamur. Singula senas alas habent, quia de fabrica tantum mundi, quae in sex diebus facta sunt, in praesenti saeculo novimus. Quod clamant ter Sanctus alter ad alterum, Trinitatis in una divinitate demonstrant mysterium. Sanctus Isidorus Hispalensis, Etymologiae, Liber VII, Caput V Source: Migne PL 82.274c

The two Seraphim whom we read about in Isaiah 1 figuratively signify the Old and New Testaments. But because they cover the face and feet of God, they signify that we cannot know the past before the world or the future after the world, but we may contemplate only the middle by their witness. Each has six wings because in this present time we know concerning the fabric of this world only those things that were made in the six days. That each cries 'Holy' three times to the other declares the mystery of the Trinity in the one Divinity. Saint Isidore of Seville, The Etymologies Book 7, Chap 5 1 Isaiah 6.2