Classifying objects --- thus collecting them into classes --- is an operation both fundamental and common in everyday life as well as in scientific endeavours. Logic, philosophy, mathematics and computer science analyse this activity by means of such notions as class', set', and type'. Furthermore, according to the so-called logical (or naive) conception of classes," these notions are related to the notion concept' in that classes are considered extensions' of concepts. This relationship, however, is a delicate one --- as we have learned from the paradoxes of class theory. The contributions...

Every human being uses signs and is involved in sign processes. Every society has developed ideas on how signs enable humans to orient themselves in their environment and deal with each other. Every language contains a rich vocabulary of words for traces, symptoms, cues and clues; for indications, utterances, and expressions; for symbols, interpretations, and models; for information, interaction, and comunication The established academic disciplines have long neglected the common properties of the various sign types on which their unified theoretical analysis could be based. The human and...