They are made by professional, semi-professional and non-professional filmmakers for a variety of reasons, most of which originate in their enthusiasm for the medium itself and their desire to develop some technical or artistic aspect of it. Who has become over a period of years the best known originator of abstract or near-abstract animation, claims that his approach to each new film is experimental in the sense that he wants to explore some new technical invention of sound and image which will expand his own particular field of work still further. The nature of his films, therefore, grows out of their own individual techniques. While most forms of animation serve the particular ends of advertising and propaganda, story-telling and entertainment, and so begin with an idea or a need that originated outside the medium, experimental films normally germinate in the heart of the medium itself. The discoveries made by the experimentalists are therefore of constant use to the professional animator because they reveal both in their success and their failure what the medium is capable or incapable of accomplishing. In a medium as free and as flexible as the drawn film, the field for experiment is endless, and it is through keeping alive this sense of experiment that animation could avoid some of the stereotyped repetitions of established forms of design and technique to which it is so often subject. Although it is obvious that a considerable degree of experiment is always possible in the course of normal commercial production, it is the "free" cinema (that is, film-making without commercial commitments) that allows those more extreme forms of work where failure or partial failure may be just as revealing as some successful new technical discovery. It is one of the limitations of the medium that the total outcome of the work, its final artistic achievement, cannot be judged in advance of its actual realization -- certainly in no sense as finally as the theatrical vitality of a play can be judged from its script or a still picture from its advance sketches. Too many essential elements -- such as visual flow and continuity, the dynamic relationship of sound and vision -- only emerge when the full work has been done. This is why an experimenter such as Len Lye who concentrates for a period on some detached aspect of animation (such as shock tactics in the use of color and sound, or the relation of optical to the drawn film) gives an invaluable service to his colleagues. The experimenter usually becomes a specialist in some chosen aspect of animation into which he researches further than anyone else until his success encourages the experiments of others. This deliberately narrow application of technical or artistic research (or both together) may, of course, lead into some cul-de-sac of the medium and then atrophy. On the other hand, the devoted work of the mathematical filmmakers, Robert A. Firethorn and Brian Salt, in animating diagrams to demonstrate geometrical propositions and other mathematical problems and formulae helped to found a limited but very important branch of mathematical film-making capable of elucidating phenomena hitherto regarded as impossible to demonstrate because they remained mental concepts in the minds of trained mathematicians.