1University of TorontoDepartment of Computer Science© Easterbrook 20041Lecture 3:What is a system? Basic Principles: Everything is connected to everything else You cannot eliminate the observer Most truths are relative Most views are complementary Defining Systems Elements of a system description Example systems Purposefulness, openness, hardness, … Describing systems Choosing a boundary Describing behaviourUniversity of TorontoDepartment of Computer Science© Easterbrook 20042General Systems Theory How scientist understand the world: Reductionism - break a phenomena down into its constituent parts E.g. reduce to a set of equations governing interactions Statistics - measure average behaviour of a very large number of instances E.g. gas pressure results from averaging random movements of zillions of atoms Error tends to zero when the number of instances gets this large But sometimes neither of these work: Systems that are too interconnected to be broken into parts Behaviour that is not random enough for statistical analysis General systems theory Originally developed for biological systems: E.g. to understand the human body, and the phenomena of ‘life’ Basic ideas: Treat inter-related phenomena as a system Study the relationships between the pieces and the system as a whole Don’t worry if we don’t fully understand each pieceUniversity of TorontoDepartment of Computer Science© Easterbrook 20043Role of the Observer Achieving objectivity in scientific inquiry1. Eliminate the observer E.g. ways of measuring that have no variability across observers2. Distinguish between scientific reasoning and value-based judgement Science is (supposed to be) value-free (but how do scientists choose which theories to investigate?) For complex systems, this is not possible Cannot fully eliminate the observer E.g. Probe effect - measuring something often changes it E.g. Hawthorne effect - people react to being studied Our observations biased by past experience We look for familiar patterns to make sense of complex phenomena E.g. try describing someone’s accent Achieving objectivity in systems thinking Study the relationship between observer and observations Look for observations that make sense from many perspectivesUniversity of TorontoDepartment of Computer Science© Easterbrook 20044RelativismTimeTime Truth is relative to many things The meanings of the words we use E.g. law of gravity depends on correct understanding of “mass”, “distance”,“force” etc The assumptions we make about context E.g. law of gravity not applicable at subatomic level, or near the speed of light E.g. Which is the step function:2University of TorontoDepartment of Computer Science© Easterbrook 20045Relativism is everywhere Truth often depends on the observer “Emergent properties of a system are not predictable from studying theparts” Whose ability to predict are we talking about? “Purpose of a system is a property of the relationship between system &environment” What is the purpose of: General Motors? A University? A birthday party? Weltanshaungen (≈ worldviews) Our Weltanshaungen permeate everything The set of categories we use for understanding the world The language we develop for describing what we observe Ethno-centrism (or ego-centrism) The tendency to assume one’s own category system is superior E.g. “In the land of the blind, the one-eyed man is king” But what use would visually-oriented descriptions be in this land?University of TorontoDepartment of Computer Science© Easterbrook 20046The principle of complementarity Raw observation is too detailed We systematically ignore many details E.g. the idea of a ‘state’ is an abstraction All our descriptions (of the world) are partial, filtered by: Our perceptual limitations Our cognitive ability Our personal values and experience Complementarity: Two observers’ descriptions of system may be: Redundant - if one observer’s description can be reduced to the other Equivalent - if redundant both ways Independent - if there is no overlap at all in their descriptions Complementary - if none of the above hold Any two partial descriptions (of the same system) are likely to be complementary Complementarity should disappear if we can remove the partiality E.g. ask the observers for increasingly detailed observations But this is not always possible/feasibleUniversity of TorontoDepartment of Computer Science© Easterbrook 20047Definition of a system Ackoff’s definition: “A system is a set of two or more elements that satisfies the followingconditions: The behaviour of each element has an effect on the behaviour of the whole The behaviour of the elements and their effect on the whole are interdependent However subgroups of elements are formed, each has an effect on the behaviourof the whole and none has an independent effect on it” Other views: Weinberg: “A system is a collection of parts, none of which can be changedon its own” …because the parts of the system are so interconnected Wieringa: “A system is any actual or possible part of reality that, if itexists, can be observed” …suggests the importance of an observer Weinberg: “A system is a way of looking at the world” Systems don’t really exist! Just a convenient way of describing things (like ‘sets’)University of TorontoDepartment of Computer Science© Easterbrook 20048Elements of a system Boundary Separates a system from itsenvironment Often not sharply defined Also known as an “interface” Environment Part of the world with which thesystem can interact System and environment are inter-related Observable Interactions How the system interacts with itsenvironment E.g. inputs and outputs Subsystems Can decompose a system into parts Each part is also a system For each subsystem, the remainderof the system is its environment Subsystems are inter-dependent Control Mechanism How the behaviour of the system isregulated to allow it to endure Often a natural mechanism Emergent Properties Properties that hold of a system, butnot of any of the parts Properties that cannot be predictedfrom studying the parts3University of
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