This paper deals with various aspects of the development of data systems for engineering materials. The problem considered here is the difference between the end-users' mental model of materials, which focuses on performance, and the concepts of properties of materials held by materials specialists. Previous treatises on this problem have elaborated on systems aspects in general, emphasising incompatibilities in the relationship mentioned and the means of overcoming these incompatibilities by service management. Another perspective applied has been the historical one, combined (...) with experience of traditional views on science and technology. The present paper pursues this general path by discussing the problem in terms of visions, illusions and myths. These phenomena are thought to be both inevitable in this context and necessary in any attempt to understand the dilemma of data systems for engineering materials. It is believed that the views of the different parties involved are to a great extent steered by visions, illusions and myths, examples of which are discussed. Furthermore, it is suggested that these phenomena have roots in, or are expressions of, paradigms and values related to world pictures. Admittedly eclectic, this discussion leads to the conclusion that a solution to the dilemma in question cannot be found within computer science, materials technology and design theory only. Guidance has to be sought in philosophy and theory of science as well as in literature and the arts. In order to make it possible to implement ideas arrived at by such means, practitioners of psychology will probably have to be consulted. (shrink)