The retina works best at a certain intensity of illumination. In bright light the nervous system contracts the pupil, and in dim relaxes it. Thus the amount of light entering the eye is maintained within limits.

When dry food is chewed, a copious supply of saliva is poured into the mouth. Saliva lubricates the food and converts it from a harsh and abrasive texture to one which can be chewed without injury. The secretion therefore keeps the frictional stresses below the destructive level.

Many more examples could be given, but all can be included within the same formula. Some external disturbance tends to drive an essential variable outside its normal limits; but the commencing change itself activates a mechanism that opposes the external disturbance. By this mechanism the essential variable is maintained within limits much narrower than would occur if the external disturbance were unopposed. The narrowing is the objective manifestation of the mechanism's adaptation.

1. A rule or procedure for solving a recurrent mathematical problem.
    
   
2. A complete, unambiguous procedure for solving a specified problem in a finite number of steps. (Richard Dorf)
    
   
3. Deterministic ALGORITHM: Given the same input information, will always produce the same output information, when applied correctly. (John Warfield)
    
   
4. Stochastic ALGORITHM: Given the same information, will not necessarily produce the same output information, even though applied correctly. (John Warfield)
    
   
6. Any mechanical or recursive computational procedure (Dictionary).

1. correspondence in some respects, especially in function or position, between things otherwise dissimilar.
    
   
2. a form of logical inference, or an instance of it, based on the assumption that if two things are known to be alike in some respects, then they must be alike in other respects.

1. A branch of philosophy dealing with values, i.e., ethics, aesthetics, religion. Based on the Greek for "worth."
    
   
2. The study of the nature of types of and criteria of values and of value judgments, especially in ethics (John Warfield)
    
   
3. The general theory of value; the study of objects of interest. (Lotze)

1. any sequence of states of a system. (Ashby, Handout, 1961)
    
   
2. The behavior of a system is overt and thus manifested in input-output relationships, whereas state trajectories are covert and must either be inferred or must be obtained by "opening the black box". (Michael Arbib)

1. The science of communication and control in animal and machine.
    
   
2. Perhaps because the field is still young, there are many definitions of cybernetics. Norbert Wiener, a mathematician, engineer and social philosopher, coined the word "cybernetics" from the Greek word meaning steersman. He defined it as the science of communication and control in the animal and the machine. Ampere, before, him, wanted cybernetics to be the science of government. For philosopher Warren McCulloch, cybernetics was an experimental epistemology concerned with the communication within an observer and between the observer and his environment. Stafford Beer, a management consultant, defined cybernetics as the science of effective organization. Anthropologist Gregory Bateson noted that whereas previous sciences dealt with matter and energy, the new science of cybernetics focuses on form and pattern.
    
   
3. A way of looking at things and a language for expressing what one sees (Margaret Mead)

1. an organism with a machine built into it with consequent modification of function;
    
   
2. an organism which is part animal and part machine. Since some theorists regard organisms as biological machines, we must define our terms further. An animal will be defined as a creature whose elements are the result of "small loop autopoiesis." That is the creature creates itself but the parts are the result of localized processes. Mind is not involved in the production of the parts. Mind results from the functioning of the parts but is manifested in the external behavior of the organism. A cyborg, then, is a creature composed of some parts constructed without the benefit of mind and some parts constructed with the benefit of mind. Furthermore the parts must be of greater than molecular size. A creature with aspirin in its body is not a cyborg. A creature with an artificial heart is a cyborg. Under this definition, animals with donated hearts, kidneys or retinas would also be cyborgs. (Umpleby)

1. A group communication structure used to facilitate communication on a specific task. The method usually involves anonymity of responses, feedback to the group as a whole of individual and/or collective views and the opportunity for any respondent to modify an earlier judgment. The method is usually conducted asyncronously via paper and mail but can be executed within a computerized conferencing environment. At the essence of the method is the question of how best to tailor the communication process to suit the situation. The Delphi method was originally developed at the RAND Corporation by Olaf Helmer and Norman Dalkey. (Murray Turoff)
    
   
2. A technique to arrive at a group position regarding an issue under investigation, the Delphi method consists of a series of repeated interrogations, usually by means of questionnaires, of a group of individuals whose opinions or judgments are of interest. After the initial interrogation of each individual, each subsequent interrogation is accompanied by information regarding the preceding round of replies, usually presented anonymously. The individual is thus encouraged to reconsider and, if appropriate, to change his previous reply in light of the replies of other members of the group. After two or three rounds, the group position is determined by averaging. (IIASA)

1. As a term in economics, demand means the amount of a commodity (good or service) that would be purchased at a given price. An associated term is [DEMAND FUNCTION,] which presents the demand-versus-price relationship. A demand function for a given commodity is compared with a corresponding [SUPPLY FUNCTION] to determine the EQUILIBRIUM PRICE: a price at which the supply offered matches the demand.
    
   
2. In another usage, demand means the amount of a commodity required for a certain purpose. It often relates to the future, as in: "the world energy demand in the year 2030 will be 35 terawatts." Implicit in this statement is that the price of energy as well as other economic conditions will be such that 35 terawatts will be consumed (purchased) if technically available. (IIASA)

     Present value =       X
                         ------
                         (l+i)n

1. In SYSTEMS ANALYSIS, the effectiveness of an ALTERNATIVE is usually represented by an aggregative expression approximating the totality of output or performance aspects of that alternative that are relevant to goal attainment. Ideally, it is a single quantitative measure that can be used to evaluate the performance level achieved in attaining the OBJECTIVES. (IIASA)
    
   
2. An absolute measure of performance. (Turoff)

1. Program A is said to be more efficient than program B if, for a given cost, a chosen aggregated measure of its positive results (such as EFFECTIVENESS or BENEFIT) is greater than that for program B. (IIASA)
    
   
2. A ratio scale measurement of a measure of performance to resources expended to obtain the level of performance. (Turoff)

1. of or relating to a process in which a sequence or sizable sample is equally representative of the whole (as in regard to a statistical parameter);
    
   
2. involving or relating to the probability that any state will recur, especially having zero probability that any state will never recur. (WEBSTER'S DICTIONARY)

1. A process of continuous change from a lower, simpler, or worse, to a higher, more complex, or better state; a process of change in some direction. (Webster's)
    
   
2. The coming into being of a new and higher order process. (Laszlo)
    
   
3. The development of each species from different, usually simpler ancestral forms. The more similar are two species, the closer in time are they likely to be to a common ancestor. (Arbib)
    
   
4. history of change in the realization of an invariant organization embodied in independent unities sequentially generated through reproductive steps, in which the particular structural realization of each unity arises as a modification of the preceding one (or ones) which, thus, constitutes both its sequential and historical antecedent. (Maturana and Varela, 1979)

1. Metaphor, that image which determines another image. (Rogers)
    
   
2. An association of a certain object(s) from one set with each object from another set (mathematics). (Rogers)
    
   
3. The normal or characteristic action of a system of entities, generally in time. (Iberall)
    
   
4. The variation of some magnitude that depends upon the variation of some other magnitude. (Iberall)
    
   
5. a notion that arises in the description made by the observer of the components of a machine or system in reference to an encompassing entity, which may be the whole machine or part of it and whose states constitute the goal that the changes in the components are to bring about. (Maturana and Varela, 1979)

In game theory one usually makes the following assumptions:

1. Each decision maker ["PLAYER"] has available to him two or more well-specified choices or sequences of choices (called "PLAYS").
    
   
2. Every possible combination of plays available to the players leads to a well-defined end-state (win, loss, or draw) that terminates the game.
    
   
3. A specified payoff for each player is associated with each end-state (a [ZERO-SUM GAME] means that the sum of payoffs to all players is zero in each end-state).
    
   
4. Each decision maker has perfect knowledge of the game and of his opposition; that is, he knows in full detail the rules of the game as well as the payoffs of all other players.
    
   
5. All decision makers are rational; that is, each player, given two alternatives, will select the one that yields him the greater payoff.

1. A structure, configuration, or pattern of physical, biological, sociological, or psychological phenomena so integrated as to constitute a functional unit with properties not derivable from its parts in summation. This German word is considered by many system thinkers (e.g., von Bertalanffy, Angyal) to convey more accurately the concept of organized wholes than the word system. (Steven Rogers)
    
   
2. The organized structure or pattern that makes up all of a person's experience of some system. This integrated view is more than the sum of the individual elements by which the field can be described. (Iberall)

1. End toward which effort is directed. (Webster's)
    
   
2. A statement, expressed in the following form: To (Action Word) (Object) (Qualifying Phrase). (John Warfield)
    
   
3. A preferred outcome in a particular situation that can be obtained within a specified time period. (Ackoff)
    
   
4. An end state consciously selected a priori. (Larry Richards)

1. Characterizing a system in which the internal parameters can be changed when necessary through feedback.
    
   
2. A heuristic idea serves as a guide for discovery. It serves as a valuable aid for empirical research but may be unproved or incapable of proof. (Umpleby)

1. A form of organization resembling a pyramid. Each level is subordinate to the one above it. See HETERARCHY. (Umpleby)
    
   
2. An organization whose components are arranged in levels from a top level down to a bottom level. (Arbib)
    
   
3. A partially-ordered structure of entities in which every entity but one is successor to at least one other entity; and every entity except the basic entities is a predecessor to at least one other entity. (Rogers)
    
   
4. Narrowly, a group arranged in order of rank or class; we interpret it to denote a rank arrangement in which the nature of function at each higher level becomes more broadly embracing than at the lower level. (Iberall)

1. Dynamic self-regulation.
    
   
2. The condition of a system when it is able to maintain its essential variables within limits acceptable to its own structure in the face of unexpected disturbances. The concept was formulated by W.B. Cannon in 1929-32.

1. the representation of ideas by graphic symbols.
    
   
2. the use of ideograms to express ideas.

1. as synonymous with CONSEQUENCE;
    
   
2. to mean any consequence (beneficial or adverse) that reaches beyond the direct purpose of a given COURSE OF ACTION, as in: "the impact of the new steel plant on employment opportunities in the region;"
    
   
3. as in (2), but the meaning restricted to adverse consequences, as in "the impact of industrial growth on the ecological environ