Lack of Evidence Reasoning

Abstract

Lack of evidence reasoning (traditional ad ignorantiam) covers three global patterns: (i) dialectical – burden of proof; (ii) epistemic – negative evidence; (iii) epistemic – »pure« absence of evidence with respect to a certain proposition. The evaluation of the first pattern depends on an interplay of epistemic, pragmatic and normative considerations which include the assesment of risk associated with our choices. A difference between (ii) and (iii) can roughly be modelled with the help of a distinciotn between external and internal negation. Absence of evidence sometimes counts as evidence of absence or negative evidence (internal negation), and sometimes as »pure« ignorance with respect to a certain propostion (external negation). A plusibilistic modus tollens, proposed by Walton offers a good reconstruction, but more factors have to be considered in telling apart good arguments from bad. I briefly mention the criterion suggested by virtue epistemology: when we in our reasoning manifest epistemic vritues (epistemic dilligence, active search for evidence, open-mindedness), the pattern is good. When we manifest epistemic vices: laziness, arrogance, boldness, close-mindedness, the pattern is bad.