Sri Aurobindo translates Brihadaranyaka Upanishad, Chapter One, Section Two, Verse 3: “Fire divided himself into three — the sun one of the three and Vayu one of the three; this is that force of life arranged triply. The east is his head and the northeast and the southeast are his arms. Now the west is his seat and the southwest and the northwest are his thighs; his sides are the south and the north; heaven is his back and the middle region is his belly; this earth is his bosom. This is he that is established in the waters wheresoever thou turn. And as that is he established who thus knoweth.”
Fire represents the creative force in manifestation. Sri M. P. Pandit describes the triple form: “And, says the Upanishad, this Godhead active in the form of Desire or Hunger that is Death, at the root of all creation divided Itself into three, each a God presiding over a different formulation of His force: as Aditya, the Sun (presiding over the Heavens of the Mind), Vayu (over the Mid-region of the Life-Force) and Agni (presiding over the Earth — the Material extension). He is not only at the base and the centre, but He stands spread out entire in the cosmic extension, manifest on each level in a different and appropriate form, but active everywhere for the same end, for advancing and self-fulfilling career of the Divine Being on the move.”
To “know” this is not an intellectual process, but a process of becoming one with this force, achieving a knowledge by identity. “And as that is he established who thus knoweth.” A knowledge by identity implies that the seeker becomes that which he has sought.
The apparently different forms and powers are all one being — this is the point of the description of the various directions related to the parts of this sacrificial horse. The separation is superficial, just as the parts of the body are all One in the creation, maintenance and ongoing activity of the being who inhabits the body.