As was explained previously, in the subsection "Defining Mediumship and Mediums", the process that takes place in the mediumship of intellectual effects involves the communicating Spirit linking its perispirit with that of the medium, so as to influence the medium to reproduce it's thought through speaking or writing.  Let's now take a look in a little more detail at how that happens.  Again, however, we will stay in general terms that relate to the basic mechanism and requirements for the transmission of thought.  Further details that are particular to specific varieties of the faculty will follow in other sub-sections.

First, we must review a few key points to keep in mind when studying the mechanism of mediumistic communication.

The above stated, there are two basic concepts that are key to understanding the mediumistic phenomena:

1. The mind is at the base of all mediumistic communications.  In all intelligent communications, the mental force of the discarnate, communicating Spirit acts upon that of the incarnate medium, and in finding, in the latter, a mental passivity or receptivity (as opposed to a block or a rejection), the communication between the two will then be possible.

2. The perispirit is the agent of all mediumistic phenomena, "which can only be produced by the reciprocal action of the fluids that are emitted by the medium and the Spirit. The development of the mediumistic faculty depends on the more or less expansive nature of the the medium's perispirit and on the greater or lesser ease of its assimilation by the Spirits."  (Kardec, Posthumous Works, #34 pg 54).

 The Basic Process = Tuning and Capturing 

The basic process of mediumistic transmission involves two principal components: A Fluidic Affinity and a Mental Tuning between the Spirit and the medium.
 

Fluidic Affinity

In order for a Spiritual manifestation to occur (both physical and intellectual), it is necessary for the perispirit of the medium to have a fluidic affinity with the perispirit of the Spirit who wishes to communicate. This is the Law of Fluidic Affinity.  

The fluidic affinity depends on the physical makeup of the medium's and the Spirit's perispirits.  Therefore, mediums can not serve as an instrument for all Spirits, for some are easily compatible and others actually repel one another.  Some mediums can only communicate with certain Spirits, or certain categories of Spirits. For this reason, there could be a Spirit present at a meeting but unable to communicate because it does not find a medium with whom it shares a fluidic affinity.  It is also for this reason that we should be skeptical of persons that claim to be able to speak with any and all Spirits.

The fluidic affinity is independent of a moral affinity; one can exist, between Spirit and medium, without the other.

As a Spirit approaches a medium with/through whom it wishes to communicate, it is also through this process of fluidic affinity that the medium, in agreement with his/her degree of sensitivity, may begin to feel the physical sensations that result from the influence of the Spirit's vibrations.  If it is a vibration of a good or elevated spirit, i.e. harmonious and benevolent, the medium will feel a sensation of happiness and a balanced emotion that envelops him/her.  Contrarily, if the Spirit is of an inferior order and thereby emits a vibration of unbalance, emotional disturbance, sickness, negativity, etc., the impressions transmitted to the medium will be unpleasant.  If the Spirit has a sensation of pain (which are actually impressions from it's abandoned physical body-- no true physical cause), the medium may capture, through vibrational tuning,  a degree of those sensations (but never to the extent that they are "felt" by the Spirit).  It is understood, however, that what the medium feels is only a spiritual impression (and not a true physical problem of his/her own) that can be controlled or lessened with proper knowledge/ training and with the help of Spiritual Benefactors, and which normally ends with the cessation of the communication (should the impression linger, the medium is helped by vibrations and passes given by the other mediums attending the meeting). 
 

Mental Tuning

It is through the process of mental tuning that the medium captures the thoughts and ideas of the communicating Spirit, to some degree of accuracy, depending on the experience and potential of the medium, the surrounding environment, and the level of affinity between the two.

How It Works

To understand how the medium is able to capture the Spirit's thoughts, remember that the perispirit of a Spirit (either incarnate or discarnate) acts as a transmitter of the Spirit's thoughts that, upon emission, travel through space, in waves, carried by the Spiritual fluids.  In the same way, the perispirit of one Spirit can act as a receiver of thought waves originating from another Spirit.  In this way, the communication of Spirits by way of thought takes place through the emission and reception of what Andre Luiz refers to as "mento-electromagnetic" waves (ie thoughtwaves).

To help us understand this process, Luiz provides us with the analogy (Ch XI) of a television transmitter and receptor.   The antenna of a television station sends out, simultaneously to many locations, signals that are encoded in electromagnetic television waves, which carry images, colors, movements, and sounds that we cannot naturally perceive. These waves can, however, be captured by a T.V. antenna and then decoded by the television set, whereby they can be perceived and understood by human beings.  In a similar way, the medium is able to capture and decode (to some degree of accuracy) the thought waves of a communicating, discarnate Spirit.  

This process can be initiated automatically, since the medium is like an antenna, more or less sensitive, capturing signals transmitted by other minds; however the tuning in for acceptance and expression of the message depends on the will of the properly educated medium who is conscious of the Spiritual influence.   (This points out an important educational need that is answered by the Spiritist Doctrine, which teaches the conscious medium how to control his/her faculty so as to exercise it in the proper time and place, and helps the unconscious medium, as well as those of us without an ostensive mediumship, to recognize the signs of spiritual influence)

Requisites to the Mental Tuning 

In addition to requiring a fluidic affinity between Spirit and medium, the ability of the Spirit and the medium to tune in to one another and establish a communication (whereby the medium is able to capture and accurately express the the thoughts of the Spirit) also requires that the thought waves of the medium and of the Spirit be in a same level of vibrational frequency, determined by their evolutionary levels.  

As each being finds itself at a certain level of [moral and intellectual] evolution, the frequency at which its thought waves vibrate- as they travel through space- varies according to its own evolutionary stage- the greater the evolution of the Spirit, the higher the frequency and strength (for traveling distances) of its thought waves. 

Because the vibrational frequency is determined by the evolutionary level of the Spirit, we refer to the vibrational compatibility of two Spirits as a moral affinity. 

What characterizes a higher level of evolution? We would find in the thoughts, intentions, and behaviors of such a being, a consistent demonstration of characteristics such as: compassion, humility, material disinterestedness, honesty, integrity, discernment, patience and understanding, resignation, desire to help others, devotion to study, dedication to work, etc. Assuming a well-evolved Spirit is present to communicate, if the medium shares in the above qualities (on a habitual basis, not just, for example, the day of a meeting), he/she would be able to establish a mental tuning with that Spirit.  To the contrary, a medium who typically demonstrates characteristics such as pride, self-interestedness, laziness, and material attachment, or even malevolent qualities of greed, hate, vengeance, etc. will only establish a moral affinity with Spirits who have those similar qualities. 

When establishing a mental tuning with an elevated Spirit, the medium's concentration and his/her state of  loosening/ expansion of the perispirit will help to elevate the medium's vibration which is weakened naturally by the influence of the material body and which, otherwise (despite the medium's respectable level of morality) still does not meet that of a discarnate (ie no influence of physical body), elevated Spirit.   In addition, the illuminated Spirit is able to increase the density of its own perispirit by absorbing subtle material from the environment, consequently lowering, temporarily, its own vibrational frequency, thus descending to the level of the medium (who can then perceive its thoughts, and with more accuracy) or similarly to that of an individual it desires to influence, such as the director of a mediumship session, a speaker teaching lessons of the Gospel, etc. (ref: M. Peralva, "Estudando A Mediunidade pg 57-62).  

The above stated, it should be pointed out that logically, the more an individual understands and practices the teachings of the Gospel, the more benevolent he/she is, the greater their natural affinity with elevated Spirits will be, thus reducing the sacrifice and work required by these renounced instructors for perception and accurate transmission of their thought by the medium.  

In the special case of a mediumship session held for giving aid to inferior or suffering Spirits, a medium will receive communications from Spirits of a lower vibrational level. You may ask then, how does a medium achieve vibrational affinity with such a Spirit? This is possible because if the medium has previously conquered/ passed a given vibrational level and is already at a better evolutionary stage, he/she, when willingly tuning in to that Spirit, is able to perceive the thoughts and emotions it wishes to express (unlike the case of communication with an elevated Spirit where the vibrations must be adjusted for the medium to perceive the Spirit's message).  In this case, the medium accepts the communication with the charitable and responsible purpose of offering the Spirit an opportunity to receive help. 

Just as our habits, thoughts and typical behaviors determine the type of Spirits that we can communicate with, they also determine the type of Spirits that we attract and that regularly accompany us.  This is the Law of Moral Affinity: Individuals of an equal moral level attract one another, and those of contrary moral levels repel.  Spirits can easily recognize our moral state by observing our vibratory levels, the appearance of our aura, the thoughts, desires and intentions that we focus on most and therefore imprint upon our perispirit, and the kinds of behaviors that we demonstrate on a daily basis.  If a medium wants to receive trustworthy messages of enlightened Spirits, and if he wants to have the company, guidance, and assistance of good Spirits, in both his daily life and in the development and practice of his mediumship, he must put forth the continuous effort to make himself worthy of such friendship and support; he must demonstrate his desire to progress, his dedication and perseverance in his work and study, and the kinds of developed moral qualities described above. And woe to the proud medium who feels he can do without the protection of good Spirits, for he is only opening the door to trickery, deception and potential wickedness of inferior Spirits who will use him for their own amusement or as an instrument in causing pain and hardship for others, as well as for himself.   The Spirit "Pascal", (in Spirit Dissertations- Ch. XXXI of "The Medium's Book") writes, "If you would receive communications from superior Spirits, you must prepare yourselves for this favor by concentration of thought, purity of intention, and sincere desire to help forward the cause of progress." 

Peralva, Martins. "Mecanismo Das Comunicações" [Mecanism of The Communications]. Estudando A Mediunidade [Studying Mediumship]. Rio de Janeiro-RJ, Brazil: Federação Espírita Brasileira [Brazilian Spiritist Federation]. 1998 (20th ed). 57-64.

Barbosa, Pedro Franco. "Principios Basicos Da Doutrina Espirita" [Basic Principles of the Spiritist Doctrine] Espiritismo Basico [Basic Spiritism]. 3rd ed. Rio de Janeiro-RJ, Brazil: Federação Espírita Brasileira [Brazilian Spiritist Federation].1987. Mediumship, pgs 143-146.

Schubert, Suely Caldas. "O Perispirito" [The Perispirit]. Mediunidade: Caminho Para Ser Feliz [Mediumship: The Path to Happiness]. Votuporanga- SP, Brazil. Casa Editora Espírita "Pierre-Paul Didier". 1999. 90

Schubert, Suely Caldas. "Sintonia E Captação" [Tuning and Capture]. Mediunidade: Caminho Para Ser Feliz [Mediumship: The Path to Happiness]. Votuporanga- SP, Brazil. Casa Editora Espírita "Pierre-Paul Didier". 1999. 104-107.

Schubert, Suely Caldas. "O Controle Das Comunicações" [Control of The Communications]. Mediunidade: Caminho Para Ser Feliz [Mediumship: The Path to Happiness]. Votuporanga- SP, Brazil. Casa Editora Espírita "Pierre-Paul Didier". 1999. 121

Rigonatti, Eliseu. "A Lei da Afinidade Fluidica" [The Law of Fluidic Affinity]. A Mediunidade Sem Lagrimas [Mediumship Without Tears]. Sao Paulo: Editora Pensamento Ltda. 2000. 46-47.

Kardec, Allan. "The Action Of The Medium In The Obtaining Of Spirit Manifestations." The Mediums' Book. 2nd ed (1st edition FEB). Trans. Anna Blackwell (translated 1876). Ed. Livraria Espírita Allan Kardec. Brasilia-DF, Brazil:  Federação Espírita Brasileira [Brazilian Spiritist Federation}. 1986. 238-253.

Kardec, Allan. "Manifestações Dos Espiritos" [Manifestations of The Spirits] Obras Póstumas  [Posthumous Works]. 8th ed. Trans. Salvador Gentile. Ed (of translation). Elias Barbosa. (translated from 6th edition in French). Araras-SP, Brazil. Instituto de Difusão Espírita [Institute for Spiritist Dissemination].1993. items 34-36, pgs 554-55.

Mecanismos Da Mediunidade- Conceitos Basicos   [Mechanisms of Mediumship - Basic Concepts] www.universoespirita.org.br/mecanismo