The Nature of the Soul:
Know, verily, that the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel. It is the first among all created things to declare the excellence of its Creator, the first to recognize His glory, to cleave to His truth, and to bow down in adoration before Him. If it be faithful to God, it will reflect His light, and will, eventually, return unto Him. If it fail, however, in its allegiance to its Creator, it will become a victim to self and passion, and will, in the end, sink in their depths.
(Baha’u’llah: Gleanings, pp. 158-159)

Verily I say, the human soul is, in its essence, one of the signs of God, a mystery among His mysteries. It is one of the mighty signs of the Almighty, the harbinger that proclaimeth the reality of all the worlds of God.
(Baha’u’llah: Gleanings, p. 160)

Verily I say, the human soul is exalted above all egress and regress. It is still, and yet it soareth; it moveth, and yet it is still. It is, in itself, a testimony that beareth witness to the existence of a world that is contingent, as well as to the reality of a world that hath neither beginning nor end.
(Baha’u’llah: Gleanings, pp. 161-162)

Consider the rational faculty with which God hath endowed the essence of man. Examine thine own self, and behold how thy motion and stillness, thy will and purpose, thy sight and hearing, thy sense of smell and power of speech, and whatever else is related to, or transcendeth, thy physical senses or spiritual perceptions, all proceed from, and owe their existence to, this same faculty. So closely are they related unto it, that if in less than the twinkling of an eye its relationship to the human body be severed, each and every one of these senses will cease immediately to exercise its function, and will be deprived of the power to manifest the evidences of its activity. It is indubitably clear and evident that each of these afore-mentioned instruments has depended, and will ever continue to depend, for its proper functioning on this rational faculty, which should be regarded as a sign of the revelation of Him Who is the sovereign Lord of all. Through its manifestation all these names and attributes have been revealed, and by the suspension of its action they are all destroyed and perish.
It would be wholly untrue to maintain that this faculty is the same as the power of vision, inasmuch as the power of vision is derived from it and acteth in dependence upon it. It would, likewise, be idle to contend that this faculty can be identified with the sense of hearing, as the sense of hearing receiveth from the rational faculty the requisite energy for performing its functions.
This same relationship bindeth this faculty with whatsoever hath been the recipient of these names and attributes within the human temple. These diverse names and revealed attributes have been generated through the agency of this sign of God. Immeasurably exalted is this sign, in its essence and reality, above all such names and attributes. Nay, all else besides it will, when compared with its glory, fade into utter nothingness and become a thing forgotten.
Wert thou to ponder in thine heart, from now until the end that hath no end, and with all the concentrated intelligence and understanding which the greatest minds have attained in the past or will attain in the future, this divinely ordained and subtle Reality, this sign of the revelation of the All-Abiding, All-Glorious God, thou wilt fail to comprehend its mystery or to appraise its virtue.
(Baha’u’llah: Gleanings, pp. 164-165)

According to the Baha’i conception, the soul of man, or in other words his inner spiritual self or reality, is not dualistic. There is no such thing, as the Zoroastrians believe, as a double reality in man, a definite higher self and a lower self. These two tendencies for good or evil are but manifestations of a single reality or self. The latter is capable of development in either way. All depends fundamentally on the training or education which man receives. Human nature is made up of possibilities both for good and evil. True religion can enable it to soar in the highest realm of the spirit, while its absence can, as we already witness around us, cause it to fall to the lowest depths of degradation and misery.
(Shoghi Effendi: Lights of Guidance, p. 208)

Progress of the Soul:
Now let us consider the soul. We have seen that movement is essential to existence; nothing that has life is without motion. All creation, whether of the mineral, vegetable or animal kingdom, is compelled to obey the law of motion; it must either ascend or descend. But with the human soul, there is no decline. Its only movement is towards perfection; growth and progress alone constitute the motion of the soul.
Divine perfection is infinite, therefore the progress of the soul is also infinite. From the very birth of a human being the soul progresses, the intellect grows and knowledge increases. When the body dies the soul lives on. All the differing degrees of created physical beings are limited, but the soul is limitless!
(`Abdu’l-Baha: Paris Talks, p. 89)

With regard to the soul of man. According to the Baha’i Teachings the human soul starts with the formation of the human embryo, and continues to develop and pass through endless stages of existence after its separation from the body. Its progress is thus infinite.
(Shoghi Effendi: Lights of Guidance, p. 204)

The Afterlife:
Know thou of a truth that the soul, after its separation from the body, will continue to progress until it attaineth the presence of God, in a state and condition which neither the revolution of ages and centuries, nor the changes and chances of this world, can alter. It will endure as long as the Kingdom of God, His sovereignty, His dominion and power will endure. It will manifest the signs of God and His attributes, and will reveal His loving-kindness and bounty. The movement of My Pen is stilled when it attempteth to befittingly describe the loftiness and glory of so exalted a station. The honor with which the Hand of Mercy will invest the soul is such as no tongue can adequately reveal, nor any other earthly agency describe. Blessed is the soul which, at the hour of its separation from the body, is sanctified from the vain imaginings of the peoples of the world. Such a soul liveth and moveth in accordance with the Will of its Creator, and entereth the all-highest Paradise. The Maids of Heaven, inmates of the loftiest mansions, will circle around it, and the Prophets of God and His chosen ones will seek its companionship. With them that soul will freely converse, and will recount unto them that which it hath been made to endure in the path of God, the Lord of all worlds. If any man be told that which hath been ordained for such a soul in the worlds of God, the Lord of the throne on high and of earth below, his whole being will instantly blaze out in his great longing to attain that most exalted, that sanctified and resplendent station….
(Baha’u’llah: Gleanings, pp. 155-156)

Know thou, of a truth, that if the soul of man hath walked in the ways of God, it will, assuredly, return and be gathered to the glory of the Beloved. By the righteousness of God! It shall attain a station such as no pen can depict, or tongue describe. The soul that hath remained faithful to the Cause of God, and stood unwaveringly firm in His Path shall, after his ascension, be possessed of such power that all the worlds which the Almighty hath created can benefit through him.
(Baha’u’llah: Gleanings, p. 161)

The world beyond is as different from this world as this world is different from that of the child while still in the womb of its mother. When the soul attaineth the Presence of God, it will assume the form that best befitteth its immortality and is worthy of its celestial habitation.
(Baha’u’llah: Gleanings, p. 157)

The nature of the soul after death can never be described, nor is it meet and permissible to reveal its whole character to the eyes of men. The Prophets and Messengers of God have been sent down for the sole purpose of guiding mankind to the straight Path of Truth. The purpose underlying Their revelation hath been to educate all men, that they may, at the hour of death, ascend, in the utmost purity and sanctity and with absolute detachment, to the throne of the Most High. The light which these souls radiate is responsible for the progress of the world and the advancement of its peoples. They are like unto leaven which leaveneth the world of being, and constitute the animating force through which the arts and wonders of the world are made manifest. Through them the clouds rain their bounty upon men, and the earth bringeth forth its fruits. All things must needs have a cause, a motive power, an animating principle. These souls and symbols of detachment have provided, and will continue to provide, the supreme moving impulse in the world of being.
(Baha’u’llah: Gleanings, pp. 156 -157)

And now concerning thy question whether human souls continue to be conscious one of another after their separation from the body. Know thou that the souls of the people of Baha, who have entered and been established within the Crimson Ark, shall associate and commune intimately one with another, and shall be so closely associated in their lives, their aspirations, their aims and strivings as to be even as one soul. They are indeed the ones who are well-informed, who are keen-sighted, and who are endued with understanding. Thus hath it been decreed by Him Who is the All-Knowing, the All-Wise.
The people of Baha, who are the inmates of the Ark of God, are, one and all, well aware of one another’s state and condition, and are united in the bonds of intimacy and fellowship. Such a state, however, must depend upon their faith and their conduct. They that are of the same grade and station are fully aware of one another’s capacity, character, accomplishments and merits. They that are of a lower grade, however, are incapable of comprehending adequately the station, or of estimating the merits, of those that rank above them. Each shall receive his share from thy Lord. Blessed is the man that hath turned his face towards God, and walked steadfastly in His love, until his soul hath winged its flight unto God, the Sovereign Lord of all, the Most Powerful, the Ever-Forgiving, the All-Merciful.
The souls of the infidels, however, shall – and to this I bear witness – when breathing their last be made aware of the good things that have escaped them, and shall bemoan their plight, and shall humble themselves before God. They shall continue doing so after the separation of their souls from their bodies.
(Baha’u’llah: Gleanings, pp. 169-171)

It is clear and evident that all men shall, after their physical death, estimate the worth of their deeds, and realize all that their hands have wrought. I swear by the Day Star that shineth above the horizon of Divine power! They that are the followers of the one true God shall, the moment they depart out of this life, experience such joy and gladness as would be impossible to describe, while they that live in error shall be seized with such fear and trembling, and shall be filled with such consternation, as nothing can exceed.
(Baha’u’llah: Gleanings, p. 171)

As to the question whether the souls will recognize each other in the spiritual world: This fact is certain; for the Kingdom is the world of vision where all the concealed realities will become disclosed. How much more the well-known souls will become manifest. The mysteries of which man is heedless in this earthly world, those he will discover in the heavenly world, and there will he be informed of the secret of truth; how much more will he recognize or discover persons with whom he hath been associated. Undoubtedly, the holy souls who find a pure eye and are favored with insight will, in the kingdom of lights, be acquainted with all mysteries, and will seek the bounty of witnessing the reality of every great soul. Even they will manifestly behold the Beauty of God in that world. Likewise will they find all the friends of God, both those of the former and recent times, present in the heavenly assemblage.
(`Abdu’l-Baha: Baha’i World Faith, p. 367)

As to the soul of man after death, it remains in the degree of purity to which it has evolved during life in the physical body, and after it is freed from the body it remains plunged in the ocean of God’s Mercy.
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.
(`Abdu’l-Baha: Paris Talks, p. 66)

It is even possible that the condition of those who have died in sin and unbelief may become changed – that is to say, they may become the object of pardon through the bounty of God, not through His justice – for bounty is giving without desert, and justice is giving what is deserved. As we have power to pray for these souls here, so likewise we shall possess the same power in the other world, which is the Kingdom of God. Are not all the people in that world the creatures of God? Therefore, in that world also they can make progress. As here they can receive light by their supplications, there also they can plead for forgiveness and receive light through entreaties and supplications. Thus as souls in this world, through the help of the supplications, the entreaties and the prayers of the holy ones, can acquire development, so is it the same after death. Through their own prayers and supplications they can also progress, more especially when they are the object of the intercession of the Holy Manifestations.
(`Abdu’l-Baha: Some Answered Questions, p. 232)

The inability of the materialistic mind to grasp the idea of the Life Eternal is no proof of the non-existence of that life.
The comprehension of that other life depends on our spiritual birth!
(`Abdu’l-Baha: Paris Talks, p. 94)

The Master [`Abdu’l-Baha] has told us that gifts and good deeds done in memory of those who have passed on, are most helpful to the development of their souls in the realms beyond….
(Shoghi Effendi: Lights of Guidance, p. 204)

Soul, Spirit, and Mind:
Just as the physical body of man is in need of its force of life, even so the human soul is in need of the divine animus and vivification emanating from the Holy Spirit. Without this vivification and sustenance, man would be an animal, nay, rather, dead.
(`Abdu’l-Baha: Promulgation of Universal Peace, p. 317)

Spirit cannot be perceived by the material senses of the physical body, excepting as it is expressed in outward signs and works. The human body is visible, the soul is invisible. It is the soul nevertheless that directs a man’s faculties, that governs his humanity.
The soul has two main faculties. (a) As outer circumstances are communicated to the soul by the eyes, ears, and brain of a man, so does the soul communicate its desires and purposes through the brain to the hands and tongue of the physical body, thereby expressing itself. The spirit in the soul is the very essence of life. (b) The second faculty of the soul expresses itself in the world of vision, where the soul inhabited by the spirit has its being, and functions without the help of the material bodily senses. There, in the realm of vision, the soul sees without the help of the physical eye, hears without the aid of the physical ear, and travels without dependence upon physical motion. It is, therefore, clear that the spirit in the soul of man can function through the physical body by using the organs of the ordinary senses, and that it is able also to live and act without their aid in the world of vision. This proves without a doubt the superiority of the soul of man over his body, the superiority of spirit over matter.
(`Abdu’l-Baha: Paris Talks, p. 86)

Now regarding the question whether the faculties of the mind and the human soul are one and the same. These faculties are but the inherent properties of the soul, such as the power of imagination, of thought, of understanding; powers that are the essential requisites of the reality of man, even as the solar ray is the inherent property of the sun. The temple of man is like unto a mirror, his soul is as the sun, and his mental faculties even as the rays that emanate from that source of light. The ray may cease to fall upon the mirror, but it can in no wise be dissociated from the sun.
(`Abdu’l-Baha: Tablet to August Forel, pp. 24-25)