Now as to what thou askest concerning the spirit and its “return” to this world of humanity and this elemental space: Know that spirit in general is divided into five sorts—the vegetable spirit, the animal spirit, the human spirit, the spirit of faith, and the divine spirit of sanctity.
The vegetable spirit is the virtue augmentative, or growing or vegetative faculty, which results from the admixture of the simple elements, with the co-operation of water, air and heat.
The animal spirit is the virtue perceptive resulting from the admixture and absorption of the vital elements generated in the heart, which apprehend sense impressions.
The human spirit consists of the rational, or logical, reasoning faculty, which apprehends general ideas and things intelligible and perceptible.
Now these “spirits” are not reckoned as Spirit in the terminology of the Scriptures and the usage of the people of the Truth, inasmuch as the laws governing them are as the laws which govern all phenomenal being, in respect to generation, corruption, production, change and reversion, as is clearly indicated in the Gospel where it says: “Let the dead bury their dead;” “That which is born of the flesh is flesh, and that which is born of the Spirit is Spirit”; inasmuch as he who would bury these dead was alive with the vegetative, animal and rational human soul, yet did Christ—to whom be glory! —declare such dead and devoid of life, in that this person was devoid of the Spirit of Faith, which is of the Kingdom of God.
In brief, for these three spirits there is no restitution or “return,” but they are subordinate to reversions and production and corruption.
But the Spirit of Faith which is of the Kingdom consists of the all-comprehending Grace and the perfect attainment and the power of sanctity and the divine effulgence from the Sun of Truth on luminous light-seeking essences from the presence of the divine Unity. And by this Spirit is the life of the spirit of man, when it is fortified thereby, as Christ saith: “That which is born of the Spirit is Spirit.” And this Spirit hath both restitution and return, inasmuch as it consists of the Light of God and the unconditioned Grace. So, having regard to this state and station. Christ announced that John the Baptist was Elias, who was to come before Christ (Matt. 11:14). And the likeness of this station is as that of lamps kindled: for these in respect to their glasses and oil-holders, are different, but in respect to their light, One, and in respect to their illumination, One; nay, each one is identical with the other, without imputation of plurality, or diversity or multiplicity or separateness. This is the Truth and beyond the Truth there is only error.
But as to the question of the Trinity, know, O advancer unto God, that in each one of the cycles wherein the Lights have shone forth upon the horizons and the Forgiving Lord hath revealed Himself on Mount Paran (Gen. 21:21, De. 33:2, Habbakkuk 3:3) or Mount Sinai, or Mount Seir (Ezekiel 35), there are necessarily three things: The Giver of the Grace, and the Grace, and the Recipient of the Grace; the Source of the Effulgence, and the Effulgence, and the Recipient of the Effulgence; the Illuminator, and the Illumination, and the Illuminated. Look at the Mosaic cycle: The Lord, and Moses, and the Fire, the Intermediary; and in the Mohammedan cycle: The Lord, the Apostle, and Gabriel. Look at the sun and its rays and the heat which results from its rays; the rays and the heat are but two effects of the sun, but inseparable from it; yet the sun is one in its essence, unique in its real identity, single in its attributes, neither is it possible that anything should resemble it. Such is the essence of the Truth concerning the Unity, the real doctrine of the Singularity, the undiluted reality as to the Sanctity.
Abdu’l-Baha’, Tablets pp. 115-118