The following are some of the most significant attributes of the Spirit, necessary to understand its immaterial nature:

IMMORTALITY - This is a quality attributed to the soul, by which it survives the death of the physical body.

INDIVIDUALITY- From the moment a Spirit is created, it maintains its individuality-- its personal characteristics, tendencies, vices and virtues, moral and intellectual advancements, etc. The eternal Spirit retains this individuality from one incarnation to the next, and throughout all the time it lives in the Spirit Realm. The inferior characteristics of a Spirit, however, are never eternal. In obedience to the Law of Progress, and aided by the infinite, education resources employed by God, each Spirit will, at its own rate, evolve and one day reach the highest level of purification.

INTELLIGENCE, THOUGHT AND REASON - Intelligence is an attribute of the Spirit, which allows it to generate thought, to reason/rationalize, and to act with premeditation. The source of intelligence is the universal intelligence. However, Spirits tell us (question # of "The Spirits' Book") that while intelligence is an attribute of each being that has it, and it constitutes the individuality of those beings, it would not be an exact comparison to say that each intelligent being draws a portion of intelligence from the universal source and assimilates it, like with the vital principle. We humans have yet to fully understand this concept. It should be noted that Instinct is a form of unreasoning intelligence, through which all beings provide for their needs (whereby it varies according to the species), even in humans where it does not diminish as intelligence grows (humans only begin to ignore it). There's no distinct line between instinct and intelligence, and they often blend; however, their actions are more distinguishable. While the actions stemming from intelligence come from thought and deliberation, those stemming from instinct are almost always spontaneous. In beings with self-consciousness and perception of outside things, instinct is allied to intelligence (i.e. to freedom of will and action).

CONSCIENCE- Reflecting the greatness and sublimity of our creator, conscience is a faculty that allows us Spirits (of both worlds) to pass moral judgment over our own thoughts and acts. It is in the conscience that the law of God is written, whereby we have the tools to discern good from bad. Since our creation as Spirits, we have forgotten or misunderstood the law as written in our conscience, and God has willed it to be recalled to our memories, through revelations, intuitive suggestions from good Spirits, and our own experiences throughout our multiple existences. Therefore, our ability of discernment depends on our level of evolution. When we incarnate in a physical body, we preserve an intuitive memory of the law, again the strength of which depends upon our moral and intellectual advancement. This memory, however, will suffer the influence of our lower instincts, and it is through the process of reincarnation that we gradually work to lift that veil of imperfections " until we reach the plentitude of psychic perfection and an integral knowledge of our own self." (quotation from the Spirit, Emmanuel, received through medium Francisco Candio Chavier in the book, "Emmanuel")

FREE WILL AND RESPONSIBILITY- Free will refers to the ability of the Spirits to follow the path that it creates for itself, making its own decisions and choosing its own acts. The discarnate Spirit enjoys that liberty when it has reached a level of evolution in which it is allowed to either chose (alone or with guidance of mentors) the trials and expiations of its next incarnation. The discarnate Spirit conserves this freedom of action in order to fight against the struggles of the material life that serve as tests of knowledge gained (trials) or as a way to make amends for past wrongdoings (expiations). The Spirit develops free-will as it gains self-awareness throughout the course of its incarnations.

Free will is a right that a Spirit holds by nature; the Spirits teach us that an alteration of our mental faculties, through a natural or accidental cause is, is the only case in which man is deprived of his free-will. However, it must be understood that Spirits do not have absolute freedom- that is, a freedom of action with no consequence. The more enlightened a Spirit's conscience is, the greater its free will, and the more accountable it will be for its actions. This is where responsibility is employed. The Spirits tell us (questions 825 and 826 of "The Spirits' Book") that we do not experience absolute freedom of action because we need one another. They explain that only a hermit in a desert enjoys such freedom, for as soon as two people find themselves together, they have reciprocal rights and duties to respect, whereby they are no longer absolutely free (to act inappropriately without suffering the results). It should also be pointed out that our evolutionary progress depends our abilities to interact appropriately with our fellow human beings, and therefore the life of a hermit does not allow for such an advancement.

While such an absolute freedom, as mentioned above, does not exist, the Spirits tell us (questions 833 and 834) that there is one condition in which we can enjoy absolute liberty, and that is in thought. They say that "God alone has full awareness of [our] thoughts and deplores or approves them according to the dictates of Divine justice."

RELATIVE POWER- This attribute refers to a power relative to that of God. You will often hear or read that Spirits are working toward perfection. But by perfection, we do not mean an absolute perfection, for that is an attribute that pertains only to God, but rather the plentitude of living and being in the divine perfection and having reached the highest stage of moral and intellectual evolution, the maximum in expansion of the conscience, whereby the Spirit identifies itself with the divine conscience.
 

Palhano Jr., Lamartine. "Espirito e Espiritualidade" [Spirit and Spirituality]. Teología Espírita [Spiritist Theology]. Rio de Janeiro: Centro Espírita Léon Denis [Leon Denis Spiritist Center]. 2001. 89-93. 

Kardec, Allan. "Elementary Principles of Spiritism." Christian Spiritism (a compilation of two Kardec books: Spiritism Reduced to Its Simplest Expression and What is Spiritism).  Trans. Allan Kardec Educational Society (translated from original French editions, published 1860 and 1859, respectively). Philadelphia, PA. Allan Kardec Educational Society. 1985. (About The Spirits), pgs 125-129.

Kardec, Allan. "Spirits." The Spirits' Book.  Trans. Allan Kardec Educational Society (translated from 2nd edition in French). Philadelphia, PA. Allan Kardec Educational Society. 1996. 31-32