Reincarnation refers to the process in which Spirits live out multiple existences in the material realm through a serious of successive rebirths into new material bodies. Each material existence provides the Spirit with the opportunity to undergo trials and/or expiations that serve as tools for its purification.

 Divine Law 

The basic notion of reincarnation or multiple existences is found in religions and philosophies throughout the history of humanity. This is not surprising since it constitutes part of the Natural Laws (created by God) that governs the universe, and of which all humans have an instinctual memory. The process of reincarnation is propelled by the Laws of Evolution, of Progression, and of Cause and Effect.

The concept of reincarnation, therefore, is not a new revelation brought by Spiritism. Spiritism only 1) increases our understanding of reincarnation by providing more complete explanations regarding its mechanism and 2) strengthens our conviction by demonstrating that the immortality of the soul and the existence of reincarnation are proven by rational philosophic arguments, spirit communications, and scientific studies.

 The Process of Evolution 

As was mentioned, evolution- the continuous and orderly progress of beings (and of worlds) - constitutes a divine law to which all of creation is subjected. But how does this process work?

To begin, we must understand that the ultimate goal in a Spirit's evolution is for it to reach a state of perfection, defined as a "condition of pure and eternal happiness" ("The Spirits' Book", #115). You might wonder why God did not create Spirits perfect to begin with, thereby avoiding all the suffering experienced along the way of gradual advancement. Well, just as we on Earth appreciate and value more that which we acquire or accomplish through hard work and struggle, the efforts of a Spirit to improve and grow allow it to earn and appreciate the supreme happiness that comes with reaching its perfection. Such progress is not possible within the limits of one single [material] lifetime, so God allows Spirits many opportunities for incarnation, so that what a Spirit does not finish in one life, it will have the chance to do so in subsequent lives.

The Spirits explain (question 115 of "The Spirits' Book") that all Spirits are created equal, in a state of simplicity and unawareness, and they are given a "mission of self-enlightenment and gradual achievement of perfection through the pursuit of knowledge and truth." The manner in which they acquire such knowledge is through the experiencing of, and successful passage through, various educational trials presented by the conditions of material life.

Spiritual Infancy

The development of a Spirit (though its lifetime once created is eternal) is similar to that of humans, in that Spirits experience a stage of infancy. It knows nothing of its origin and has no experiential knowledge of, or predisposition toward bad and good. Its intelligence is in a state of potentiality, only beginning to unfold. At this stage, the spirits acts in the material world, motivated largely by instinct, and in pursuit of physical survival.

The Learning Begins

As the Spirit begins to interact with its environment, its knowledge increases. Though still very limited, the Spirit is now endowed with the beginnings of its free will, that is, the ability to choose its actions. Spirits are tested by various external elements, including those inherent to material life as well as exposure to the influence of less advanced spirits (both incarnate and discarnate) that try to tempt the Spirit and lead it astray. Such influences are present at any phase of development (though as a Spirit progresses it becomes less vulnerable); God is just however, and Spirits, again at all phases, are also aided by the influence of good spirits that encourage them with positive influence.

Free will, then, offers the Spirit a chance to resist negative influences and temptations through the decisions that it makes. Those who choose well take a step forward with each good decision and will advance quicker than those who choose poorly.

Progress Continues

The Spirit that began in a state of simplicity and unawareness gradually begins to climb the first steps on the evolutionary ladder. As it advances, it will develop emotions, which indicate activity and self-consciousness, and its free will develops in accordance with its level of progress, which allows the spirit to have merit for its progress.

Reaping the Harvest

Action and Reaction is a natural law that makes a Spirit accountable for its actions. For every action that a Spirit takes, both good and bad, it will experience the effects accordingly. And as a Spirit's knowledge (both self-awareness and knowledge of God's law) and free will grow, the Spirit's responsibility/accountability increases.

Kardec explains ("The Spirits' Book", discussion to question #191) that a Spirit's multiple existences and the opportunities they offer for progression resemble a series of days in a person's life, "in each of which one acquires new experience and knowledge", and that "just as there are unprofitable days in a human life, there are also incarnate lives in the existence of a spirit that are unrewarding because the spirit did not use them well."

The moments and lifetimes in which we do not use our opportunities well-buy using our free will in such a way that hurts or violates the rights of another, thereby acting against God's will, we create debts - or as some say "bad karma" for which we must make amends. If we do not do so within the same lifetime, natural law will make it necessary for us to do so in another life, through some form of suffering that serves as an expiation and a learning tool for our Spirit. Such suffering is not punishment from God, but is rather the manifestation of natural law and is actually self imposed by our own conscience that, upon awareness of its wrongdoing, inflicts upon the Spirit a tormenting guilt from which it seeks relief, obtained through the acceptance of this suffering with submission and resignation. In this way, the opportunity to undergo such afflictions is actually a blessing from our creator. In addition to a resigned acceptance, positive actions and charitable behavior will help the spirit to ease its suffering.

Between each material life, the Spirit spends an even greater amount of time in its true home, the Spiritual realm, where it has the opportunity to analyze its behaviors in its last life, recuperate and re-energize, study, and prepare for its next incarnation. As the Spirit progresses from one life to the next, it will continue to make amends for past mistakes, while most likely creating new debts- though less severe- along the way, whereas it will proceed to continue "making marks on its balance sheet", so to speak, until it is eventually able to eliminate all debts and negative tendencies, demonstrating only virtuous qualities. In this way, we are all the masters of our own destiny, meaning the amount of suffering we must endure and the speed at which we progress.

Trials, Expiations, and Missions

Throughout the process of its evolution, a Spirit's incarnations will offer it experiences for trials, expiations, and in the case of highly evolved spirits, a special mission. Trials allow the Spirit to put its learning and knowledge to the test. They can come in both the form of difficult experiences that test, for example one's humility, honesty, resignation or compassion, or in the form of apparent blessings such as wealth or talents to test whether or not the Spirit will use it to help others. As mentioned above, it is through expiations and the sufferings- both of physical and spiritual nature- either chosen or imposed, that the Spirit may make amends for past mistakes. Lastly, in some cases, the Spirit may incarnate with a mission, by which it is charged with some task to complete in benefit of the progression of humanity.

Additional Considerations

Progression by Law
By law, all Spirits, though at their own pace, will eventually progress through all phases. A spirit, out of laziness or ill-intentions, may remain stationary for some period, but, thanks to God's merciful support, it never remains in that state permanently. Likewise, Spirits never regress in their evolution . The Spirits tell us (#s 118 of The "The Spirits' Book") that "as [Spirits] advance, they understand the things that slow their progress. When a Spirit finishes a trial, it learns a lesson it will never forget."

Always the Right Road
Question # 120 of the "The Spirits' Book" asks, "Do all spirits have to taste evil to arrive at good?" The Spirits answered, "not evil, but unawareness" meaning that a Spirit who uses its free will to make good choices will progress much faster, but nevertheless, as Kardec later comments (discussion #127), "even those who follow the right road from beginning do not attain a state of perfection immediately because they still have to acquire the knowledge and experience that goes with this state." He compares this to the development of a child, whereby even the most precocious, mature, and well-behaved child, must still pass through youth to reach adulthood. Spirits who follow the right road will be rewarded, however, for they will arrive sooner at perfection; their imperfections will be fewer and their suffering will be less.

Neutrality vs. Perfection 
The elimination of a Spirit's negative tendencies and "clearance of all debts" represent a significant amount of progress. However, it is not enough for the Spirit to reach a point where it simply does no harm to another. The state of true perfection is one in which a Spirit has completely developed virtuous and altruistic behaviors- when it is motivated by love and its actions are driven not by self-interest but rather in the interest of the well being of others.

Number of Incarnations 
There is no set number, or limit to the number, of incarnations that a Spirit must undergo, though, Kardec writes, ("The Spirits' Book"" discussion #191) "A spirit needs, from our point of view, an immense length of time to pass from spiritual infancy to spiritual adulthood, i.e., the state at which its progress is complete." Each Spirit, by the use of its free will, ultimately determines the speed of its own progress through each required stage.