According to their own Spirits, these " are the smart principle personalizations, as the bodies are personalizations of the principle material. The time and the mode of such training are unknown ".  we are all Spirits, but we live here on earth we need unity between spirit and matter, so we can give life to the latter. The spirits are the intelligent beings of creation, having been created by God ignorant and simple, is to say, without knowledge and without consciousness of good and evil, being the intelligence, therefore, an essential attribute of the spirit and not the smart principle.
The spirits inhabit the universe in various stages of evolutionary development and in various categories of worlds. Are equipped with skills to acquire intellectual knowledge and morals through reincarnations, and thus the perfection and purity.
By appropriate, it is bring up the considerations of the spirit Raniranee about what is the spirit, where asserts that d set logging the spirit in all its particularities, the moment wouldn't be possible, because it would bring us some confusion as a result of being spiritual, children still learning now the first letters about life, which is difficult to define the life to those who remain in death.
Remember that the incarnate and much of the disembodied, not knowing even the physical body and its numerous functions, such as want to meet the other bodies used by the spirits and, even more, knowing his own mind? It is necessary to start the Earth to be possible, then, to feel and see the sky.
Claims to be the forces of limitless Spirit, blossoming slowly on soul who learns to use their potential and that the definition of the spirit, as many want to, is not being denied by God, being given only as the means and the needs of souls, by numerous intermediate works and messages brought by mediumship, in Brazil and in the world.
The spirits are the spirit world or of intangible intelligences, being the preexisting material spiritual world. The spirit is not divisible, is a complete individuality and, although it is not palpable, it's not nothing, because nothing does not exist, being a quintessenciada matter and no analogy for us, so ethereal that cannot be perceived by our senses.
Kardec elucidates that are said to be spirits immaterial, because, by your essence, differ from everything we know under the name of matter, but that in fact we are like the blind regarding the essence of superhuman beings and that only we can define using always imperfect comparisons or for a effort of the imagination. 
The spirit, although progress through the countless reincarnations, evolves also in the spiritual plane, in erraticidade. It acquires special knowledge which could not acquire on Earth, being corporeal State and the spiritual state two forms of progress that develop solidarity. It is for this reason that the spirit passes alternatively by these two modes of existence.
 KARDEC, Allan. The book of spirits. 93 ed. Brasília: FEB. 2013. Q. 79. p. 86.  the spirits book reviewed by the spirit Raniranee. Available at: <https://bit.ly/2tEIAxF>. Access in: 26 jun 2018.  KARDEC, Allan. The book of spirits. 93 ed. Brasília: FEB. 2013. Q. 82. p. 86.