Singpurwalla addresses the important challenge of modelling a unique individual. He proposes “propensity” as an approach to describing the reliability or life time of “one of a kind”. My view is that mathematical modelling is only possible when we assume that nonunique features provide sufficient information for statistical prediction to be useful. As far as possible, we should test our assumptions. However, contrary to a popular perception of Hume, we always rely on some beliefs.
"What Does “Propensity” Add?." Statist. Sci. 31 (4) 549 - 551, November 2016. https://doi.org/10.1214/16-STS585