In a certain sense human activity systems do not exist, only perceptio — Peter Checkland

"A root definition describing a notional system chosen for its relevance to what the investigator and/or people in the problem situation perceive as matters of contention."

"The concept of action research arises in the behavioural sciences and is obviously applicable to an examination of human activity systems carried out through the process of attempting to solve problems. This core is the idea that the researcher does not remain an observer outside the subject of investigation but becomes a participant in the relevant human group. The researcher becomes a participant in the action, and the process of change itself becomes the subject of research. In action research the roles of researcher and subject are obviously not fixed: the roles of the subject and the practitioner are sometimes switched: the subjects become researchers... and researchers become men of action."

"We reduce the complexity of the variety of the world in experiments whose results are validated by their repeatability, and we may build knowledge by the refutation of hypotheses."

"In an unrestricted science such as biology or geology, the effects under study are so complex that designed experiments with controls are often not possible. Quantitative models are more vulnerable and the chance of unknown factors dominating the observations is much greater."

"The core of a root definition of a system will be a transformation process (T), the means by which defined inputs are transformed into defined outputs. The transformation will include the direct object of the main activity verbs subsequently required to describe the system."

"Cursory inspection of the world suggests it is a giant complex with dense connections between its parts. We cannot cope with it in that form and are forced to reduce it to some separate areas which we can examine separately. .."