The Art of Abduction
We&#x27;ve been working on abduction at our institute for a while. My take on it is that there is no abduction as a logical inference. Although he portrayed it as a third type of inference in some of his writings, C.S. Peirce thought the same, he emphasized many times that abduction is indeed an art and ultimately a psychological phenomenon.<p>The problem with abduction is that it doesn&#x27;t have a justificatory value on a par with those of deduction and induction. Deduction forms the basis of mathematics and is basic enough to be intuitively acceptable when you learn logic. In turn, induction is mostly justified deductively. Finite discrete probability theory is purely deductive. Under the <i>assumption</i> that a process is stochastic (e.g. that a die is fair when it is thrown), probabilities follow deductively. Probability theory for infinite and uncountable domains are straightforward generalizations. There are only few leaps of faith when doing <i>inductive inference</i>, (1) when you transfer conclusions from a smaller to a larger population, (2) apply the law of large numbers, or (3) make reasonable guesses about an underlying probability distribution.<p>There is no such justification for abduction. Even just hypothesis generation, the narrow sense of &quot;abduction&quot;, is way more complicated than induction and deduction. Theory selection, the broader aspect of &quot;abduction&quot; that has been labeled &quot;inference to the best explanation&quot; since Harman&#x27;s horrible 1965 paper, is even more complicated. Both IBE and abduction require and involve deductive and inductive reasoning, but there is no indication that they can be fully justified deductively and inductively. The best you may get is an account that describes some form of theory discovery as a logical or probabilistic description of a supposed cognitive mechanism, and then the discovered theory might be proved deductively or confirmed inductively.<p>Defenders of abduction have claimed that we select hypotheses not just on the basis of their likelihood but also based on other properties like theory virtues (&quot;loveliness&quot; in Lipton&#x27;s terms). That might be true but doesn&#x27;t concern the justificatory value of the inference. In the end, the only hypotheses we&#x27;re interested in (and should be interested in, from an epistemic point of view) are the ones that are most likely to be true. Hence, theory virtues, loveliness, plausibility, etc., all just have a justificatory value insofar as they increase the subjective probability that a hypothesis is true, and in that respect they are really just heuristics.<p>Some Bayesian takes on abduction and IBE get this right and I have nothing against them. I just wanted to mention that many, if not the majority of philosophers of science do not consider abduction a third type of inference and fairly skeptical about it, for the above reasons and a few more.