Instead of the formal definition, a list of general characteristics of agents will be given. Together these characteristics give a global impression of what an agent "is". 
The first group of characteristics, which will be presented in section
2.2.1, are connected to the weak notion of the concept "agent". The fact that an agent should possess most, if not all of these characteristics, is something that most scientists have agreed upon at this moment.
Thus, a simple way of conceptualising an agent is as a kind of UNIX-like software process, that exhibits the properties listed above. A clear example of an agent that meets the weak notion of an agent is the so-called softbot (`software robot'). This is an agent that is active in a software environment (for instance the previously mentioned UNIX operating system).
Agents that fit the stronger notion of agent usually have one or more of the following characteristics: 
Although no single agent possesses all these abilities, there are several prototype agents that posses quite a lot of them (see section 3.2.2 for some examples). At this moment no consensus has yet been reached about the relative importance (weight) of each of these characteristics in the agent as a whole. What most scientists have come to a consensus about, is that it are these kinds of characteristics that distinguish agents from ordinary programs.
What exactly makes an agent "intelligent" is something that is hard to define. It has been the subject of many discussions in the field of Artificial Intelligence, and a clear answer has yet to be found.
[WOOL95] for a more eleborated overview of the theoretical and practical aspects of agents.
 See: Casterfranchi, C (1995). Guarantees for autonomy in cognitive agent architecture. In Woolridge, M. and Jennings, N. R., ed., Intelligent Agents: Theories, Architectures, and Languages (LNAI Volume 890), page 56-70. Springer-Verlag: Heidelberg, Germany.
 See: Genesereth, M. R. and Ketchpel, S. P. (1994). Software Agents. Communications of the ACM, 37(7): page 48-53.
 Note that the kind of reactivity that is displayed by agents, is beyond that of so-called (UNIX) daemons. Daemons are system processes that continuously monitor system resources and activities, and become active once certain conditions (e.g. thresholds) are met. As opposed to agents, daemons react in a very straight-forward way, and they do not get better in reacting to certain conditions.
 Analogous to the "sleep" state in a UNIX system, where a process that has no further tasks to be done, or has to wait for another process to finish, goes into a sleep state until another process wakes it up again.
 See: Shoham, Y. Agent-oriented programming. Artificial Intelligence, 60(1): page 51-92, 1993.
 See, for instance, Bates, J. The role of emotion in believable agents. Communications of the ACM, 37(7): page 122-125, 1994.
 See: Maes, P. Agents that reduce work and information overload. Communications of the ACM, 37(7): page 31-40, 1994.
 See, for instance, Norman, D. How Might People Interact with Agents. In Communications of the ACM, 1994 issue, Juli 1994.
 This list is far from complete. There are many other characteristics of agents that could have been added to this list. The characteristics that are mentioned here are there for illustrative purposes and should not be interpreted as an ultimate enumeration.
 See: White, J. E. Telescript technology: The foundation for the electronic marketplace. White paper, General Magic Inc., 1994.
 See: Rosenschein, J. S. and Genesereth, M. R. Deals among rational agents. In Proceedings of the Ninth International Joint Conference on Artificial Intelligence (IJCAI-85), page 91-99, Los Angeles, United States, 1994.
 See: Galliers, J. R. A Theoretical Framework for Computer Models of Cooperative Dialogue, Acknowledging Multi-Agent Conflict. PhD thesis, page 49-54, Open University, Great Britain, 1994.
 See: Eichmann, D. Ethical Web Agents. Proceedings of the Second International World-Wide Web Conference. Chicago, United States, October 1994.
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