Abstract
In this paper, the traditional view that argumentum ad ignorantiam is a logical fallacy is challenged, and lessons are drawn on how to model inferences drawn from knowledge in combination with ones drawn from lack of knowledge. Five defeasible rules for evaluating knowledge-based arguments that apply to inferences drawn under conditions of lack of knowledge are formulated. They are the veridicality rule, the consistency of knowledge rule, the closure of knowledge rule, the rule of refutation and the rule for argument from ignorance. The basic thesis of the paper is that knowledge-based arguments, including the argument from ignorance, need to be evaluated by criteria for epistemic closure and other evidential rules that are pragmatic in nature, that need to be formulated and applied differently at different stages of an investigation or discussion. The paper helps us to understand practical criteria that should be used to evaluate all arguments based on knowledge and/or ignorance.
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Notes
The most famous example of a system that uses the closed world assumption is PROLOG, an artificial intelligence system that has negation as failure.
The argument diagram above was constructed using the diagramming tool Araucaria (Reed and Rowe, 2002). It can obtained free at this site: http://www.computing.dundee.ac.uk/staff/creed/araucaria/.
Capital letters A, B, ..., refer to statements (propositions), entities that are true or false.
Girle (2003, p. 110) calls this axiom the veridicality principle.
There is a large philosophical issue here of how ‘knowledge’ should be defined. Many epistemologists would say that this defeasible sense of the term is merely true belief. The term ‘knowledge’ as used in computing, and science generally, often has an honorific sense, referring to what is accepted by the scientific community at any given time.
Once again, the term ‘knowledge’ often seems to have an honorific meaning in science. See note 6.
By not putting in the subscript to the K-operator as done in the set of axioms above, we here ascend to a higher level of abstraction where some set of rational agents is presumed to be constant.
Once again the question is raised of whether knowledge can be abstracted from the agents that are held to possess it. The thesis above relativizes knowledge to a community of knowers. But who are they? Are they individual agents, the community of scientists, or the general population? I shall make no comment on this philosophical question.
This example is similar to one given by Collins et al. (1975, p. 398).
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Walton, D. Rules for Reasoning from Knowledge and Lack of Knowledge. Philosophia 34, 355–376 (2006). https://doi.org/10.1007/s11406-006-9028-6
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DOI: https://doi.org/10.1007/s11406-006-9028-6