  Diagrammatic, analogical or iconic representations are often contrasted with
linguistic or logical representations, in which the shape of the symbols is
arbitrary. The aim of this paper is to make a case for the usefulness of
diagrams in inferential knowledge representation systems. Although commonly
used, diagrams have for a long time suffered from the reputation of being only
a heuristic tool or a mere support for intuition. The first part of this paper
is an historical background paying tribute to the logicians, psychologists and
computer scientists who put an end to this formal prejudice against diagrams.
The second part is a discussion of their characteristics as opposed to those of
linguistic forms. The last part is aimed at reviving the interest for
heterogeneous representation systems including both linguistic and diagrammatic
representations.
