Nuno David, Jaime Simão Sichman and Helder Coelho (2005)
The Logic of the Method of Agent-Based Simulation in the Social Sciences: Empirical and Intentional Adequacy of Computer Programs
Journal of Artificial Societies and Social Simulation
vol. 8, no. 4
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Received: 02-Oct-2005 Accepted: 02-Oct-2005 Published: 31-Oct-2005
Figure 1. Programs and languages as models, according to Fetzer (1999). The series of three dots stands for the possible existence of compilers and interpreters that effect some causal connection between programs/machines at different levels.
'The Schelling model assumes that a family will move only if more than one third of its immediate neighbours are of a different type (e.g., race or ethnicity). The result is that very segregated neighbourhoods form even though everyone is initially placed at random, and everyone is somewhat tolerant.' (Axelrod, 1997a, p.24)
…the often pretheoretical assumption that computers are formal. In one way or another, just about everyone thinks that computers are formal — that they manipulate symbols formally, that programs specify formal procedures, that data structures are a kind of formalism, that computational phenomena are uniquely suited for analysis by formal methods (…) this antisemantic thesis is contradictory to the tacit recognition that computation, in one form or another, is a symbolic or representational or information-based or semantical phenomenon — in other words, an intentional phenomenon. Though in ways we do not understand, the states of a computer can model, or simulate, or represent, or stand for, or carry information about, or signify other states in the world.
'The basic units of the model are ten actors arranged on a line. The actors can be thought of as independent political units, such as nations…The basic cycle of the model is called a year. In each year, three actors are chosen one after another at random to become active…The selection of actors to be active is based upon the notion that ambitious leaders and potential disputes arise at random…If B fights rather than pays…' (1997b, p.128, our italics)
'The broad aim of this research is to begin the development of a more unified social science, one that embeds evolutionary processes in a computational environment that simulates demographics, the transmission of culture, conflict, economics, disease, the emergence of groups, the emergence of groups, and agent coadaptation with an environment, all from the bottom up.' (1996, p.19)
'In the beginning, a small population of agents is randomly scattered about a landscape. Purposeful individual behaviour leads the most capable or lucky agents to the most fertile zones of the landscape: these migrations produce spatially segregated agent pools. Though less fortunate agents die on the wayside, for the survivors life is good: food is plentiful, most live to ripe old ages, populations expand through sexual reproduction, and the transmission of cultural attributes eventually produces spatially distinct 'tribes.' But their splendid isolation proves unsustainable…' (1996, p. 8, our italics)
• Select a neighbouring agent at random;
• If the neighbour is fertile and of the opposite sex and at least one of the agents has an empty neighbouring site (for the baby), then a child is born;
• Repeat for all neighbours.
"We offer this definition of 'friendship' as a simple local rule that can implemented efficiently, not as a faithful representation of current thinking about the basis for human friendship."
'We do not think we would have been able to divine it. But that really is all that is happening.' (1996, p.52)
Figure 3. A possible scenario in the culture dissemination model of Axelrod (1997b). Agents interacting at iteration n are marked with a square. Marked numbers are the cultural features that interact.
'There is a critical value for parameter C [the minimum proportion of like-coloured agents], such that if it is above this value the grid self-organises into segregated areas of single colour counters. This is lower than 0.5' (Edmonds, 2003, p.123).
'Even a desire for a small proportion of racially similar neighbours might lead to self-organised segregation' (p.123, our italics).
'…researchers who adopt this 'third discipline' often report that they have experienced difficulty in having research results accepted within traditional peer reviewed journals.'
2 There are several perspectives, which sometimes seem to rely on contradictory positions. See, among others, Axelrod (1997a), Byrne (1997), Conte et al. (1997), David et al. (2004), David et al. (2005), Epstein (1999), Epstein and Axtell (1996), Gerhenson (2002), Gilbert (1995), Gilbert and Troitzsch (1999), Goldspink (2002), Gross and Strand (2000), Kluver (2003), Troitzsch (1997). See also subscript number 3.
3 See, specifically, Kluver et al. (2003, paragraph 5.5). For an epistemological perspective based on the classical theory of computation, a la Turing-Church, see Epstein (1997). Conversely, this article attempts to present an alternative — and possibly incompatible — perspective.
4 The classical theory of computation is also known as the Turing-Church thesis. See also subscript number 7.
5 See, e.g. Dijkstra (1976).
6 As Fetzer remarks (1999), high-level languages may not even be related to any physical machine, when appropriate compilers and interpreters are not provided. For example, the role of the programming language presented in Dijkstra (1976) is to reason about programs without even mentioning their behaviour on real computers. Compiling details are left unstudied.
7 If we accept Turing-Church thesis, according to the classical theory of computation, all computation that terminates can be simulated by a first-order language. The understanding of the computational process according to higher-order logics finds apparent insuperable obstacles (see e.g. Papadimitriou, 1994).
8 This conception does not necessarily suggest or imply the conception of artificial intelligence as to the use of the intentional stance for modelling artificial agents. Those are relatively independent issues.
9 The code is made accessible on the Web for consultation.
10 For example, an agent with the culture given by the quintet 23637 means that the first feature has value 2, the second attribute has value 3 and the fifth attribute has value 7.
11 Epstein and Axtell (1996,p.73) call it tag-flipping.
12 The Hamming distance between two binary strings is obtained by comparing the strings position-by-position and totalling the number of positions at which they are different.
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