1. Mediate association has been well known since the time of Hamilton, who was the first to determine its nature and to give a personal example that has become classic. Loch Lomond recalled to him the Prussian system of education because, when visiting the lake, he had met a Prussian officer who conversed with him on the subject. His general formula is this: _A_ recalls _C_, although there is between them neither contiguity nor resemblance, but because a middle term, _B_, which does not enter consciousness, serves as a transition between _A_ and _C_. This mode of association seemed universally accepted when, latterly, it has been attacked by Muensterberg and others. People have had recourse to experimentation, which has given results only in slight agreement.[23] For my own part, I count myself among those contemporaries who admit mediate association, and they are the greater number. Scripture, who has made a special study of the subject, and who has been able to note all the intermediate conditions between almost clear consciousness and the unconscious, considers the existence of mediate association as proven. In order to pronounce as an illusion a fact that is met with so often in daily experience, and one that has been studied by so many excellent observers, there is required more than experimental investigations (the conditions of which are often artificial and unnatural), some of which, moreover, conclude for the affirmative. This form of association is produced, like the others, now by contiguity, now by resemblance. The example given by Hamilton belongs to the first type. In the experiments by Scripture are found some of the second type--e.g., a red light recalled, through the vague memory of a flash of strontium light, a scene of an opera. It is clear that by its very nature mediate association can give rise to novel combinations. Contiguity itself, which is usually only repetition, becomes the source of unforeseen relations, thanks to the elimination of the middle term. Nothing, moreover, proves that there may not sometimes be several latent intermediate terms. It is possible that _A_ should call up _D_ through the medium of _b_ and _c_, which remain below the threshold of consciousness. It seems even impossible not to admit this in the hypothesis of the subconscious, where we see only the two end links of the chain, without being able to allow a break of continuity between them. 2. In his determination of the regulating causes of association of ideas, Ziehen designates one of these under the name of "constellation," which has been adopted by some writers. This may be enunciated thus: The recall of an image, or of a group of images, is in some cases the result of a sum of predominant tendencies.