For a long time, people have been trying to characterize or define the notion of system. After all, systems are supposed to be what System Theory is about. The results so far have been contradictory and unsatisfactory. This confusion at the foundations has led many to conclude that there is no such thing as a "system" and hence to deny that System Theory is about anything. Even those most sympathetic to the notion have difficulties at this level. The very founders of System Theory did not try to say what a system was; and as for System Theory, they characterized it only obliquely, by saying it comprised all studies of interest to more than one discipline. They thereby begged the entire question.
p. 1 - "Some comments on systems and system theory," (1986)