The following is an excerpt from "The Mediums' Book", Chapter X, 'The Nature of Spirit Communications'.

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" If we bear in mind the infinite variety that exists among Spirits, under the double aspect of intelligence and of morality, we perceive that a corresponding diversity must exist in their communications, which necessarily reflect the elevation or the backwardness of the Spirits by whom they are made; and that the quality of their ideas, their degree of knowledge or of ignorance, their virtues and their vices, will be evidenced by the communications which emanate from them, and which no more resemble each other than do, among men, those of the savage and the most enlightened European.  But all the shades presented by Spirit-communications may be grouped into four principal categories; according to their most salient characteristics, they may be designated as coarse, frivolous, serious, and instructive.

Coarse communications are those which shock propriety.  They can only proceed from Spirits at a very low stage of development, still soiled with the impurities of materiality.  They are in no way different from the utterances of coarse and vicious men, and are revolting to every one who has the least delicacy of sentiment;  being trivial, obscene, insolent, arrogant, malevolent, or even impious, according to the character of the Spirits who make them.

Frivolous communications emanate from Spirits who are frivolous, mischievous, and keen to trickery, rather than wicked, and who attach no importance to what they say.  As there is nothing unseemly in their communications, they amuse persons who take pleasure in futile talk.  Such Spirits, however, sometimes come down upon their interlocutors with clever, biting sallies, throwing off, in the midst of commonplace jokes, hard truths which often hit the mark.  These frivolous Spirits are around us in swarms, and take every opportunity of mixing themselves up with the communications of other Spirits.  Having no respect for veracity, they take a mischievous pleasure in hoaxing those who have the weakness or presumption to believe in them.  Those who amuse themselves with such communications naturally give access to foolish and deceptive Spirits;  while, at the same time, they repel serious ones, who, like serious men, avoid the society of the unreasoning and the giddy.

Serious communications are distinguished by the gravity of the subjects of which they treat, and by the seriousness of their tone.  Every communication exempt from coarseness and frivolity, and having in view a useful object, even though it be of merely private interest, may be classed as 'serious', but nevertheless, it may not be exempt from error.  Serious Spirits are not all equally enlightened;  there are many things of which they are ignorant, and concerning which they may be mistaken.  For this reason, Spirits who are really superior constantly advise us to submit all communications to the test of examination and reason.

It is therefore imperatively necessary to distinguish between communications that are seriously true and communications that are seriously false.  This is not always easy, for it is often under the guise of seriousness that presumptuous and superficial Spirits seek to foist upon us their erroneous ideas and absurd theories, unscrupulously assuming the most honored and even the most venerated names, in order the more effectually to impose upon us, a method of deception which constitutes one of the most dangerous stumbling blocks of practical Spiritism .....

Instructive communications are those which are not only serious, but also convey the teachings of Superior Spirits on points of science, morality, philosophy, etc.  They are more or less profound in proportion to the elevation and dematerialization of the communicating Spirit.  In order to reap benefit from such communications, they must be followed up with regularity and perseverance.  Serious Spirits attach themselves to those who desire instruction, and assist them, but those who see in these manifestations a source of passing amusement are left by them to the companionship of Spirits as frivolous as themselves.  It is only by the regularity and frequency of such communications that we can appreciate the moral and intellectual worth of the Spirits with whom we thus hold conversation, and the degree of confidence which they deserve.  If experience is necessary in order to form a true estimate of men, it is, if possible, still more necessary in forming a true estimate of Spirits.

In applying to these communications the qualification of instructive, we imply that they are true; for what is not true can never be instructive, though expressed in the most imposing language.  We therefore do not include in this category certain teachings which have nothing serious about them but their high-flown and pretentious style by which Spirits possessed of more presumption than philosophy endeavor to mislead.  But such Spirits, being unable to disguise their shallowness are incapable of keeping up their false assumptions for any length of time; they soon betray their weak side if we continue to converse with them and ply them with questions which compel them to show their incompetence. "