An Introduction to General Systems Thinking

is an excellent book by Jerry Weinberg. Like many of Jerry's books it also has a repeat snippet page here. As usual I'm going to quote from a few pages:
As Leonardo observed, "the boundary of one thing is the beginning of another." thinkers use the term "interface" to describe the part of the world that, like the two-faced god Janus, looks both inside and outside at the same time.
...for everything has a fringe of involvement with its surroundings (quoting P.W. Bridgman)
Systems, on the average, are more tightly connected than the average. is better to analyze in terms of doings or happenings than in terms of objects or static abstractions (quoting P.W. Bridgman)
And with this shift in time there occurs a shift in the entity of concern - from an object, a pattern of matter in space, to a behavior, a pattern of events in time. [R.W.Gerard]
Why do things stay the same?
How can we know the dancer from the dance? [William Butler Yeats]
Although we may identify the system by its functions, this is only a convenience, for the "real" identity lies in "the permanence of the relations among the component parts" - the "structure". (quoting Herbert Spencer).
The more sure we are, the more likely we are to suffer an illusion.
To be a successful generalist, one must study the art of ignoring data and of seeing only the "mere outlines" of things.
If something explains everything, it explains nothing.