Autonomous agents have become an influential and powerful paradigm in a great variety of disciplines, from sociology and economics to distributed artificial intelligence and software engineering to philosophy. Given that the paradigm has been around for awhile, one would expect a broadly agreed-upon, solid understanding of what autonomous agents are and what they are not. This, however, is not the case. We therefore join the ongoing debate on what are the appropriate notions of autonomous agency. We approach agents and agent ontology from a cybernetics and general systems perspective, in contrast to the much more common in the agent literature sociology, anthropology and/or cognitive psychology based approaches. We attempt to identify the most fundamental attributes of autonomous agents, and propose a tentative hierarchy of autonomous agents based on those attributes.