We develop a logical modelling approach to describe evolvable computational systems. In this account, evolvable systems are built hierarchically from components where each component may have an associated supervisory process. The supervisor's purpose is to monitor and possibly change its associated component. Evolutionary change may be determined purely internally from observations made by the supervisor or may be in response to external change. Supervisory processes may be present at any level in the component hierarchy allowing us to use evolutionary behaviour (...) as an integral part of system design. We model such systems in a revision-based first-order logical framework in which supervisors are modelled as theories which are at a logical meta-level to the theories of their components. This enables evolutionary change of the component to be induced by revision-based changes of the supervisor at the meta-level. In this way, the intervention required in evolutionary change is modelled purely logically. The hierarchical component-based structure is fairly intricate so we present the basic ideas firstly in a simple setting, the well-known blocks world, before introducing tree-based structures to represent component hierarchies. We also introduce some techniques for establishing the behaviour of evolvable systems specified in this logical framework. The ideas and concepts are driven by example throughout. We conclude with a more substantial example, that of a simple model of an evolvable system of automated bank teller machines. (shrink)

The “preferential entailments” considered in this text are all defined in the same way, by a binary relation, or “preference relation”. This relation can be among interpretations, or sets of interpretations, or among “states” which are copies of interpretations or copies of sets of interpretations. This provides four kinds of preferential entailments. What we do here is to provide a characterization result for these four kinds of preferential entailments. We choose properties as simple and natural as possible, and sometimes we (...) provide various characterizations for the same notion. It appears that the apparently most complicated notion possesses by far the simplest characterization result. A by-product of our results is that case is equivalent to case : we can define directly the relation among sets of interpretations, eliminating the need for “states” in this case. The paper deals mainly with propositional logic, however it describes also the situation in first order logic. Any complete theory has one model in the propositional case and many models in the first order case. This is why the simplest notion in the first order case has exactly the same syntactical characterization as the notion in the propositional case. (shrink)

A conservative extension of first order logic, suitable for circumscriptive inference, is introduced.