In a PDP network, patterns of activity across a collection of nodes can often be interpreted as representing a concept: Concepts are represented in a distributed fashion. Individual connections cannot be interpreted as representing a conceptual relation. The connections between the nodes in a PDP network are just excitatory or inhibitory. The whole matrix of connections encodes what the network knows about a given problem. Change the connections and you change the knowledge. In contrast to a semantic network, it is impossible to add a completely new fact to the network's knowledge base simply by the addition of new nodes or connections. Individual facts are represented by collections of nodes and connections, often involving the entire network. New information has to be integrated with old information. Conversely, it is usually impossible to remove a single fact from the network simply by the deletion of a node or a connection.