An alternative to Logic as a representation formalism is semantic networks and frames.
The property list feature of Lisp was used to represent networks of nodes connected by labeled arcs, forming semantic networks, by M. Ross Quillian, Robert F. Simmons, and others.
Following Minsky's ``frame paper'', procedures and inheritance were added to semantic networks to form frame systems.
More recently, frame systems have been interpreted as having semantics similar to logic, and they have been seen as more efficient implementations of certain common types of reasoning.