I was reading `Abdu’l-Bahá’s explanation of the nature of the soul and found the following passages most enlightening.
From the moment the soul leaves the body and arrives in the Heavenly World, its evolution is spiritual, and that evolution is: The approaching unto God.
In the physical creation, evolution is from one degree of perfection to another. The mineral passes with its mineral perfections to the vegetable; the vegetable, with its perfections, passes to the animal world, and so on to that of humanity. This world is full of seeming contradictions; in each of these kingdoms (mineral, vegetable and animal) life exists in its degree; though when compared to the life in a man, the earth appears to be dead, yet she, too, lives and has a life of her own. In this world things live and die, and live again in other forms of life, but in the world of the spirit it is quite otherwise.
The soul does not evolve from degree to degree as a law—it only evolves nearer to God, by the Mercy and Bounty of God.
The soul is not a combination of elements, it is not composed of many atoms, it is of one indivisible substance and therefore eternal. It is entirely out of the order of the physical creation; it is immortal! In the world of spirit there is no retrogression. The world of mortality is a world of contradictions, of opposites; motion being compulsory everything must either go forward or retreat. In the realm of spirit there is no retreat possible, all movement is bound to be towards a perfect state. ‘Progress’ is the expression of spirit in the world of matter. The intelligence of man, his reasoning powers, his knowledge, his scientific achievements, all these being manifestations of the spirit, partake of the inevitable law of spiritual progress and are, therefore, of necessity, immortal.
Question.—What is the difference between the mind, spirit and soul?
Answer.—It has been before explained that spirit is universally divided into five categories: the vegetable spirit, the animal spirit, the human spirit, the spirit of faith, and the Holy Spirit.
The vegetable spirit is the power of growth which is brought about in the seed through the influence of other existences.
The animal spirit is the power of all the senses, which is realized from the composition and mingling of elements; when this composition decomposes, the power also perishes and becomes annihilated. It may be likened to this lamp: when the oil, wick and fire are combined, it is lighted; and when this combination is dissolved—that is to say, when the combined parts are separated from one another—the lamp also is extinguished.
The human spirit which distinguishes man from the animal is the rational soul, and these two names—the human spirit and the rational soul—designate one thing. This spirit, which in the terminology of the philosophers is the rational soul, embraces all beings, and as far as human ability permits discovers the realities of things and becomes cognizant of their peculiarities and effects, and of the qualities and properties of beings.
But the human spirit, unless assisted by the spirit of faith, does not become acquainted with the divine secrets and the heavenly realities. It is like a mirror which, although clear, polished and brilliant, is still in need of light. Until a ray of the sun reflects upon it, it cannot discover the heavenly secrets.
But the mind is the power of the human spirit. Spirit is the lamp; mind is the light which shines from the lamp. Spirit is the tree, and the mind is the fruit. Mind is the perfection of the spirit and is its essential quality, as the sun’s rays are the essential necessity of the sun. This explanation, though short, is complete; therefore, reflect upon it, and if God wills, you may become acquainted with the details.