Modelling rhetorical legal “logic”—a double syllogism

This paper looks at legal reasoning from the point of view of the work of the lawyer, rather than the law itself. In the case of Common Law systems, this means a more flexible view of how tasks are divided between the humans and the computer system, with an emphasis on decision support rather than complete automation. A process-based model of the lawyer's work is proposed in the form of a double syllogism, which displays an aesthetically pleasing symmetry, but also a significant asymmetry in the role played by perceived precedents. This arises from the use of inductive, rather than deductive, reasoning. The potential complications arising from the issue of the perception of precedents are discussed in depth.The double-syllogism model is then considered in the light of CommonKADS terminology and models. It is suggested that decision support systems using knowledge-based techniques, as required to support lawyers working under Common Law jurisdiction, raise a stronger form of the interaction problem that is well known in knowledge-based systems. This means that such systems are not well catered for in the existing CommonKADS Organisational, Agent, Task and Communication Models. The double-syllogism model is suggested as a supplement to CommonKADS in the development of such systems, at least until a more generic addition is available.