A Theory of Similarity and Uncertainty

The paper calls attention to the ontological and epistemic roots of uncertainty, and emphasizes the plurality of grounds that could make evidence uncertain and induction tentative. In particular, we have seen that the sources of uncertainty are manifold due to the interplay of circumscription difficulty and epistemic complexity. However, it is precisely this lack of determinacy that allows the handling of uncertainty by means of a principle of non-exclusion. This is because the domain of similarity relations coincides with the partial overlap of the ontological and epistemic sets. The relationship between similarity and likelihood implies that multiple orders of likelihood are also possible: any given situation may be less or more likely depending on which particular order we are considering. In short, the domain of knowable uncertainty is constrained both on the ontological and the epistemic side. Within that domain, uncertainty allows similarity relationships and permits the assessment of likelihood. Multiple orders of likelihood may be associated with different degrees of rational belief as they are not all founded on an equally solid knowledge basis. However, as we have seen, multiple overlaps of likelihood orders may be possible. It is reasonable to conjecture that confidence in the likelihood assessment for any given situation would increase if a variety of different orders of likelihood were to assign the same likelihood assessment for that particular situation. For example, it is reasonable to think that situations associated with different orders of likelihood for the short- and the long-term would in fact be associated with strongest likelihood confidence in the case of cross-over points. In short, uncertainty at its most fundamental level is associated with co-existence of different orders of similarity and likelihood. This co-existence makes it very difficult to assess particular situations, as they might look respectively likely or unlikely depending on which features are considered. Clearly this difficulty may be due to the way in which any given situation is circumscribed, or to the categories available to make sense of existing circumscriptions. However, different likelihood orders may sometimes intersect one another (see above). This means that the very plurality of uncertainty dimensions that makes it difficult in general to assess any given situation, may turn out to be an advantage when facing the special circumstances in which the same assessment of the situation in view is grounded in a plurality of different orders of likelihood.