Meaning of the term “soul” in various kingdoms of creation -- 'Abdu'l-Baha explains

As to the question concerning the soul, know thou, verily, that "soul" is a term applied to numerous realities, according to the exigencies of the following relations in regard to development in the world of existence:

(1) In the mineral kingdom, soul is called "latent force," silently working for the disintegration of the substance of the mineral.

(2) In the vegetable kingdom it is called “virtue augmentative," or the power of growth, which attracts and absorbs the delicate materials of inorganic substance found in the mineral kingdom of matter, and transforms them into the condition of growth. Thus the inorganic substance found in the mineral kingdom becomes growing vegetable life through the effect of the word of God. This vegetable soul, i. e. "virtue augmentative," or power of growth, is a quality which is produced by the admixture of elements, and appears in accidental organisms, of which contingency is an essential attribute.

(3) In the animal kingdom it is called "sense perceptions" (or instinct). This soul term, as applied to the animal kingdom, is also a natural quality resulting from the mixture of the elements, and it appears from their mingling and combination, for it is a quality which results from the composition of bodies (organisms) and is dispersed at their decomposition. From this we are to understand that the animal soul is not endowed with the capacity of attaining immortality, as the life force is dispersed at the decomposition of the animal tissues.

All these things up to this point are a contingent reality, and are not a divine reality. But a contingent reality, which is perpetuated by the fullness of existence, will then suffer no corruption, and will thus become a divine reality, for the accidental reality is only distinguished from the existent reality by its subjection to corruption. For transformation is an essential necessity to every contingent reality, and this is what the mature wisdom has deemed advisable. 

(4) In the human, worldly soul signifies the "rational being, or mind." This has a potential existence before its appearance in human life. It is like unto the existence of a tree within the seed. The existence of the tree within the seed is potential: but when the seed is sown and watered, the signs thereof, its roots and branches, and all of its different qualities, appear. Likewise, the "rational soul" has a potential existence before its appearance in the human body, and through the mixture of elements and a wonderful combination, according to the natural order, law, conception, and birth, it appears with its identity.

Be it known that to know the reality or essence of the soul of man is impossible, for, in order to know a thing, one must comprehend it, and since a thing cannot comprehend itself, to know one's self in substance or essence is impossible. As the comprehended cannot be comprehended, man cannot know himself in reality or essence. In order to obtain knowledge of any reality, or soul of man, the student must study the manifestations, qualities, names and characteristics of man. This much can be stated, that the reality of man is a pure and unknown essence constituting a depository, emanating from the Light of the Ancient Entity God. This essence or soul of man, because of its innate purity, and its connection with the unseen Ancient Entity, is old as regards time, but new as regards individuality. This connection is similar to that of the ray of the sun the effect to the primal cause. Otherwise, the thing that is generated, or the creature, has no connection with or relation to its Generator or its Creator.

Since the pure essence, whose identity is unknown, possesses the virtues of the worlds of matter and of the Kingdom, it has two sides -- first, the material and physical; second, the mental and spiritual -- which are attributes not found as qualities of matter. It is the same reality which is given different names, according to the different conditions wherein it becomes manifest. Because of its attachment to matter and the phenomenal world, when it governs the physical functions of the body, it is called the human soul. When it manifests itself as the thinker, the comprehended, it is called the mind. And when it soars into the atmosphere of God, and travels in the spiritual world, it becomes designated as spirit.

There are two sides to man. One is divine, the other worldly; one is luminous, the other dark; one is angelic, the other diabolic; man is equal to the animals in all sensuous conditions, for all animal characteristics exist in him. Likewise, divine and satanic qualities are contained in man; knowledge and ignorance; guidance and error; truth and falsehood; generosity and avarice; valour and timidity; inclination towards God and tendency towards satan. Chastity and purity; corruption and vileness; economy and avidity; good and evil; all are contained in man.

(5) If the angelic side becomes more powerful, and the divine power and brightness surround man, then the second birth takes place, and eternal life is found at this point. Man becomes then the noblest among creatures. On the other hand, if sensuous qualities surround, and if terrestrial darkness and sensuous passions predominate, if they meet in man only the worldly feelings, if they find him a captive of evil qualities and fallen into everlasting death, then such a man is the basest and most abject among all creatures.  In such a man, divine power does not exist. An animal is not considered unjust and evil because of its cruelty and injustice, for it is not endowed, as is man, with divine qualities; but if man falls into the same evil condition, it is evident that he has permitted his ungodly attributes to overcome the divine qualities with which he was endowed. This shows the baseness and meanness that exist in human nature. 
(Words of Abdul Baha from Diary of Mirza Ahmad Sohrab, February, 1914; Star of the West vol. 7, no. 19, March 2, 1917; Baha’i Scriptures, pp. 403-405)