There are many ideas behind Agile and Scrum, and I daresay that Sutherland and Schwaber could not accurately remember in 1999 all of these ideas nor where they came from. People are easily influenced, and forget where the influences came from.
Scrum is a very interesting mixture of very simple ideas (e.g., KISS or Keep It Stupid Simple) and complex ideas (e.g., Complex Adaptive Systems ideas).
Scrum was invented before the Agile Manifesto and Agile Principles were articulated in 2001. Still, Ken Schwaber and Jeff Sutherland were there at Snowbird in 2001. They would say that Scrum ‘follows’ those agile principles. So, see, read, and think about those specific ideas.
Scrum was also strongly influenced by many other ideas. Early on, maybe before any experimental teams, they read one particular Harvard Business Review article. So, one set of ideas comes from the New New Product Development Game article by Takeuchi and Nonaka. Here are the six ideas described there:
I cannot too strongly recommend that article and almost any article or book by Takeuchi and Nonaka.
Let me mention again the Complex Adaptive Systems ideas. See this link in Wikipedia for a start.
I am convinced that Peter Drucker’s idea about knowledge workers, and similar related ideas had a significant impact. Related are Takeuchi and Nonaka’s (and others’) ideas around knowledge creation.
In my opinion, Scrum is mainly about people. That is, it is an attempt to get smart people working together much more effectively. So, embedded in Scrum are many ideas about people and how they work, and how they might work together better.
Agile is sometimes thought of as being overly optimistic about people. Not so, in my opinion.
Scrum is not overly-optimistic. It does not assume that people are always perfect; quite the contrary. Scrum seems quite well aware of all the weaknesses in human nature, and that humans have strengths but also weaknesses. Examples: We are easily distracted and we tend to procrastinate. So, Scrum does some things to address these likely issues. On the other hand, Scrum is somewhat positive, in that it assumes that usually people can work together effectively, and become more effective together.
Schwaber talks a lot about the empirical process. That is, in our work we should not use a defined theoretical modeling approach (waterfall), but rather an empirical process that requires and supports us in inspecting (using transparency) and adapting quickly.