Abstract: A transactional computer system comprises a plurality of entities including at least one entity of each of the following forms, a Thing entity having the properties of identifying a client system and uniquely identifying an object in that client system, a Proposal entity for defining a transaction, the Proposal entity being subordinate directly or indirectly to a Thing entity and having the properties of modelling at least one external agent to carry out a transformation in relation to the first entity, and a Decision entity capable of communicating with a Proposal entity and having the properties of defining the types of decision that may be made, and determining the responses in relation to those decisions.