2. Peirce on Induction and Abduction
Abduction, on Peirce’s mature view, differs in kind from induction.
Induction, for Pierce, includes any use of empirical evidence to test or
support a hypothesis. Abduction, by contrast, is reasoning which
introduces hypotheses into inquiry in the first place: “abduction
commits us to nothing. It merely causes a hypothesis to be set down upon
our docket of cases to be tried” (Pierce 1932-58,
§5.602).1 Abduction, in other words, aims to identify
hypotheses which are worth pursuing further (McKaughan 2008). As
Peirce stresses, the pursuit-worthiness of a hypothesis cannot be
reduced to how plausible or probable it is; in fact: “Sometimes the
very fact that a hypothesis is improbable recommends it for provisional
acceptance on probation [i.e. for further testing]” (Peirce
1932-58, §6.533). For instance, if a hypothesis can be reliably
falsified and if it would be valuable (given the goals of inquiry) to
rule it out conclusively, increasing its improbability can make
it more pursuit-worthy (cf. Nyrup 2015).
While Peirce thus distinguishes abduction from induction, he is somewhat
ambiguous as to what it means for abduction to “introduce” or
“suggest” hypotheses. Does it mean generating hypotheses or
merely to select already generated hypotheses for further
pursuit?2 Sometimes it is characterized in terms of
generation. For instance: “Abduction consists in studying facts anddevising a theory to explain them”; it is “the process
of forming an explanatory hypothesis” (1932-58, §5.145, §5.171,
emphases added). However, abduction is also described as “the operation
of adopting an explanatory hypothesis” (§5.189, emphasis added),
through the characteristic inference schema (ibid. ):
The surprising fact, C ¸ is observed;
But if A were true, C would be a matter of course,
Hence, there is reason to suspect that A is true.
In order to employ this inference schema (as Peirce himself notices) a
reasoner must already have formulated the explanatory hypothesisA mentioned in the second step, seemingly contradicting the idea
that abduction is the process through which A is generated.
This tension can be resolved by interpreting abduction as encompassing
both the generation and the selection of hypotheses to be
pursued.3 The reason it makes sense to include both
under the label ‘abduction’ is that they share a common goal, namely to
identify hypotheses worth pursuing further. As Fann (1970, 41-43)
argues, simply formulating new hypotheses is not difficult. Rather,
since we cannot examine every conceivable hypothesis, the problem is to
generate hypotheses it is worth spending time and resources considering
further. Generative reasoning should thus be evaluated along the same
dimensions as reasoning concerned with selecting hypotheses for pursuit.
The above schema outlines one salient criterion for pursuit-worthiness,
namely whether the proposed hypothesis accounts for the phenomena we
seek to explain. But other criteria—e.g. testability and economy—are
relevant too; the schema does not exhaust what Peirce meant by abduction
(Pietarinen and Bellucci 2014, 355-6).
This interpretation also allows us to answer two objections, often
raised against normative accounts of generative
reasoning.4 First, since we do not control which ideas
occur to us, how can anything normative be said about hypothesis
generation? The answer is that one can still choose when ,whether and how to generate new ideas, and these choices
can be normatively evaluated in terms of how efficiently they generate
pursuit-worthy hypotheses. For example, a rather ineffective strategy
would be to flip through a medical lexicon, hoping to chance upon
diseases with symptoms similar to the ones observed in the patient.
Experienced physicians will (one hopes) be able to deploy better
strategies for generating hypotheses. We discuss some of these in the
clinical case in Section 4.
Second, why assume that there is any particular kind of reasoning (viz.
“abduction”) through which physicians generate hypotheses? The answer
is that we do not assume this. For us, the term ‘abduction’ does not
refer to any particular process of reasoning. Rather, the distinction
between abduction and induction refers to a difference in goals:
abduction aims to identify hypotheses worth pursuing further; induction
aims to determine the likelihood of a hypothesis. Similarly, although
the generation and selection of hypotheses for pursuit share a normative
foundation, the concrete (e.g. verbal, mental, symbolic) reasoning
processes involved in generating or formulating a hypothesis of course
differ from those involved in choosing between already formulated
hypotheses. Trying to overcome the problem of having too fewhypotheses is still different from trying to overcome the problem of
having too many . The two can still be distinguished.
A particularly useful, unified interpretation of abduction, which has
been proposed in recent scholarship (Hintikka 1998; Paavola 2004;
Pietarinen and Bellucci 2014), construes abduction in terms ofstrategic reasoning . On this interpretation, individual choices
about hypothesis generation and pursuit should be evaluated in terms of
whether they contribute to an overall investigative strategy. We will
explain this interpretation in more detail and show how it helps make
sense of our clinical case study in Section 6.
In summary, we draw the following lessons from these Peircean ideas.
First, we distinguish three general types of reasoning in medical
diagnosis: (1) generating plausible diagnostic hypotheses; (2) selecting
and prioritizing these for pursuit, i.e. for further testing and
consideration; (3) accepting or rejecting diagnoses, or more broadly,
evaluating the how likely different diagnoses are in light of the
available evidence. Second, we take (1) and (2) to share the same goal,
viz. pursuit-worthiness. Third, thinking about diagnosis in terms of
strategic reasoning provides a useful unified framework for evaluating
the generation and selection of hypotheses. We will now describe in more
detail how these lessons apply to medical diagnosis.