The Water-Lily Pond, 1899 (Oil on Canvas), by Claude Monet.
...it would be impossible to go through a regular course of experimentation, in spiritism, as we do in physics or chemistry. In the natural sciences, we operate on brute matter, manipulating it at will, and with almost a certainty of producing a given effect; in spiritism, on the contrary, we have to deal with intelligences who have their liberty. (A. Kardec, , Paragraph 31)
Mediumship is an important issue in Spiritism. In fact, the Spiritist knowledge – understood as a body of principles exposed and justified in the main Spiritist work, “The Spirit’s Book” by A. Kardec – was only possible with the discovery that the human senses could be extended to unveil the hidden reality of the spirit world.
In a series of posts, we present the basics of mediumship in a way stripped down to its essential principles . This is necessary because, since the first rigorous treatment of the subject in "The Medium's Book" , mediumship has been befouled by several misconceptions that have arisen from the human tendency towards the supernatural and the mystery.
In this sense, the word mediumship expresses the original meaning with much more virtue than modern terms such as "channeling" or "psychic power", for example. These terms, in fact, shroud the subject in an atmosphere of mystery and alienation which is incompatible with Kardec's original intention of use and which is clearly mandatory if one should approach all these matters with objectivity and rigor. It is time for many spirit believers to leave the terrain of phantasy to understand properly mediumship as a simple phenomenon of Nature.
One can define mediumship as "the faculty of mediums"  which leads us to the meaning of the word "medium". In the "Spiritist Vocabulary of The Medium's Book", Kardec defines a medium as
Medium (Latin medium, intermediary). Any person who serves as an intermediary between spirits and men.
However, from a practical standpoint, the meaning of the word medium has been extended to encompass a variety of ideas. As put forward by Kardec already in the XIXth century , one can broadly identify two acceptions corresponding to a connotation arising from the original Kardec's denotation:
Restricted meaning: refers to every person endowed with a mediator power which manifests itself in the form of physical effects or in the transmission of the spirits' thoughts in written or spoken forms.
From such specific meaning, people come up with a variety of ideas and interpretations. The most common one is that every person in a broad sense is a medium:
General meaning: Any person who is able to receive or transmit communications from spirits is a medium regardless the degree the faculty.
From such general meaning, the slightest spirit influence is interpreted as mediumship and the person in case is dubbed a 'medium' or 'psychic'. However, such reasoning is a mistake.
The statement 'we are all mediums' should be understood in the general meaning , in other words, that all people are somewhat subjected to the influence of the spirits without implying everyone is a medium in the proper or accurate sense.
Such semantic difference should not confuse us because it is an essential linguistic phenomenon manifested in many words: 'mediumship' and 'medium' are in fact polysemic words.
We limit ourselves here to the restricted meaning in accordance to which mediums are persons able to keep communication with the spirits or serve as an instrument between them and incarnate people. At the present level of our knowledge, the answer about the nature of mediumship is embodied in some statements found in many Kardec's books (as explained by the spirits) which link the faculty to the human organism. For example:
The faculty depends on a special organic disposition which is susceptible to development. 
We have seen persons who were utterly incredulous write on first holding a pencil; while fervent believers, not possessing the germ of the medianimic faculty, have failed to do so in spite of protracted efforts on their part, which proves that the faculty depends on the organism. 
...this aptitude results from a physical disposition nevertheless. 
Mediumship is not related to anything else in spite of many contrary popular beliefs: in particular, it does not depend on intelligence or on the moral qualities  nor on the philosophical or religious beliefs of the medium.
Therefore, we cannot deny the existence of mediums in environments or contexts that are highly uncorrelated to Spiritualist beliefs. In particular, mediums exist in spite of unqualified moral behavior of the medium and his/her poor scholar level. Because mediumship is independent of any belief or moral condition, no one can become a medium by simply believing in or adhering to Spiritualism.
Therefore, mediums are found in all walks of life as a consequence of the physiological nature of the human body resulting in a modification of the human ordinary senses. Mediumship is a natural occurrence, possibly disseminated uniformly across all social strata. As such, mediumship has no relation whatsoever with the physical, mental or spiritual unbalances of the person.
Another consequence of the mediumship dependence on the organism is the phenomenon of mediumship fading. As the body becomes old, many mediums lose their abilities. Another possible change in mediumship is in the way it manifests itself. Many mediums who have started as physical mediums see their faculties become more intelligent as they dedicate themselves to more intellectual mediumship practices [8, q. Kardec]. Such a change in the way the faculty manifests is an example of mediumship development.
Since mediumship can improve with time - or, on the contrary, fade away with aging - the question about mediumship training arises naturally. First, two important key points are worth considering:
- No one who was not born a medium can become a medium;
- Since the faculty is linked to the spirits (as external agents), the higher the purpose of it, the more flexible, consistent and notorious the manifestations will be.
The first point implies that mediumship can be trained unless its principle exists as explained in the following excerpt from The Medium's Book :
They would be equally mistaken who should expect to find in this work a universal and infallible recipe for making mediums; for, although every one possesses the germ of the qualities necessary for becoming a medium, those qualities exist in very different gradations, and their development depends on causes which no one can control by his own will alone. The rules of poetry, painting, and music, do not make poets, painters, or musicians, of those who are not gifted with genius, although those rules guide men in the employment of the faculties which they naturally possess. So it is with the work before us; its object is to indicate the means of developing the medianimic faculty so far as the receptivity of each will permit; and, above all, to guide it in a manner that may elicit its usefulness. Not, however, that this is the sole end for which the present work has been undertaken. (Italics added)
It is, therefore, a huge mistake and a waste of time to force at all costs the appearance of a faculty in a person whom no principle can be found [9b]. More recently, some spirit authors  have stated similar opinions
Can one provoke the development of mediumship?
Mediumship is hardly a fruit of precipitation no matter the field of action of the medium. It demands the most indispensable spontaneity, because every mediumship practice is directly controlled by mentors on the spiritual plane.
The same applies therefore to 'developing psychic abilities' which, in the lack of its seeds, cannot be achieved. But, how do we know a person has the mediumship seeds? In Paragraph 200 of , Kardec makes the important remark
It is found in children and in old people, in men and in women, independently of health, or of intellectual and moral development ; it has nothing to do with a person's temperament; there is but one way of ascertaining its existence, viz., by actual experiment.
Likewise, Emmanuel  emphasizes the importance of spontaneity in mediumship:
No one can force the development of whatever faculty because, on this matter, every spontaneity is crucial. If the spontaneous eruption of mediumship is observed in its most gentle expression, we should accept it with good will and the best intention of work. (Italics added)
It is important to recognize however that mediumship is an additional dimension that may affect possibly the individual behavior or state of health. On the other hand, a common mistake is to believe someone is a medium because he or she manifests some kind of mental imbalance or disease. These require special medical, psychological or spiritual treatment according to the case. In particular, mediumship practice should be avoided in the presence of physical problems such as contagious diseases or when the organic state of the medium can be impaired by strong emotions. In the specific case of mental or spiritual unbalances, the mediumship practice cannot be initiated or should be discontinued.
There is no definitive or sufficient collection of methods or techniques able to prompt mediumship development. Rather, the fair application and practice of a spontaneously manifested faculty are the only guarantee for its proper development . Being a medium also presupposes a total commitment to serious study and continuous effort in reshaping one's moral action in accordance with superior guidance. Since the aim of mediumship is spirit communication, mediumship group attendance is recommended as a way of study and experimentation. However, the same recommendations one can make for an individual apply to an entire group. Hence, serious mediumship groups are more indicated for people who really wish to develop mediumship fully.
Finally, as for the limits to mediumship development, they are defined by the direction of the mediumship work given by its recipient. The higher (in the ethical sense) the purpose, the more flexible and notorious will be the mediumship manifestation and degree attained. Since mediumship is a collaborative work between two planes of life, the medium cannot purport to be the only origin of its extension and force. The limits are therefore laid down by the spirits which, under certain circumstances, can severely limit the medium's work. In  we find, for example, an interesting communication by Socrates which distinguishes itself by its severity:
When the germ of a faculty exists, it always shows itself by unequivocal signs. By keeping to his own specialty, a medium is more likely to obtain useful and satisfactory results; he who tries to do everything, does nothing well. The desire to enlarge indefinitely the circle of one's medianimic faculties is a vainglorious pretension which will not be allowed to go unpunished; good spirits always abandon the presumptuous, who thus become the sport of liars. It is, unfortunately, no rare thing to see mediums discontented with the gifts they possess, and aspiring, from vanity or ambition, to the possession of exceptional faculties, which might bring them into prominence; a pretension which robs them of their most precious quality, that, viz., of being safe mediums.
The invocation of 'punishment' in the quote above is simply a figure of language. In practice, good or learned spirits turn away from bad mediums naturally because, with time, the proper conditions of spirit interchange are lost. Among the causes of loss is pride, when mediums trust too much his/her abilities. Because the faculty depends on the organism, many bad mediums retain their 'psychic abilities' which become useless tools.
Therefore, mediums should always consider mediumship as a gift that can be "taken away" if the practice is misguided because the good fruits of mediumship depend on collaborative work with good spirits. History of Spiritualism is full of examples of misguided mediums.
Next: Fundamentals of mediumship II (types)
Next: Fundamentals of mediumship II (types)
References and notes
 These notes are based on the article "Estudos sobre mediunidade" by S.S. Chibeni and C. S. Chibeni available in Portuguese only.
 See, for example, the first citation of the name mediumship (in French, mediumnité) in Chapter
 A. Kardec. The medium's book. Transl. A. Blackwell. 2nd Ed. by the Brazilian Spiritist Federation, 1986.
Everyone who is in any degree influenced by spirits is, by that very fact, a medium.
 A. Kardec.Instruction Pratique sur les Manifestations Spirites. (Practical instructions on spirit manifestations). According to the 10th Edition. Union Spirite Française et Francophone. 1923.
Cette faculté tient à une disposition organique spéciale susceptible de développement.
 Ref. 3, Paragraph 209.
 Ref. 3.Paragraph 94.Question 5.
 Ref. 3. Answer to Question 19, Paragraph 223:
Mediumship, properly speaking, is independent of the intelligence as well as of the moral qualities.
 As a modern example in Brazil, many mediums initially debuting as physical mediums, have become healing mediums. In this case, the faculty is still in the domain of the physical manifestations, but its focus was changed to healing people.
[9b] Ref. 3. Paragraph 198.
 Emmanuel. O Consolador. (Medium Francisco C. Xavier.). Question 383, 8th Ed. by the Brazilian Spiritist Federation, 1940.
 Ref. 3, II Part, Chapter XVI. End of Paragraph 198.