Casuistry is reasoning used to resolve moral problems by extracting or extending theoretical rules from particular instances and applying these rules to new instances. The term is also commonly used as a pejorative to criticize the use of clever but unsound reasoning (alleging implicitly the inconsistent—or outright specious—application of rule to instance), especially in relation to moral questions (see sophistry).
The agreed meaning of "casuistry" is in flux. The term can be used either to describe a presumably acceptable form of reasoning or a form of reasoning that is inherently unsound and deceptive. Most or all philosophical dictionaries list the neutral sense as the first or only definition. On the other hand, the Oxford English Dictionary states that the word "[o]ften (and perhaps originally) applied to a quibbling or evasive way of dealing with difficult cases of duty." Its textual references, except for certain technical usages, are consistently pejorative ("Casuistry‥destroys by Distinctions and Exceptions, all Morality, and effaces the essential Difference between Right and Wrong"). Most online dictionaries list a pejorative meaning as the primary definition before a neutral one, though Merriam-Webster lists the neutral one first. In journalistic usage, the pejorative use is ubiquitous
While a principle-based approach might claim that lying is always morally wrong, the casuist would argue that, depending upon the details of the case, lying might or might not be illegal or unethical. The casuist might conclude that a person is wrong to lie in legal testimony under oath, but might argue that lying actually is the best moral choice if the lie saves a life. (Thomas Sanchez and others thus theorized a doctrine of mental reservation, which developed into its own branch of casuistry.) For the casuist, the circumstances of a case are essential for evaluating the proper response.
Typically, casuistic reasoning begins with a clear-cut paradigmatic case. In legal reasoning, for example, this might be a precedent case, such as premeditated murder. From it, the casuist would ask how closely the given case currently under consideration matches the paradigmatic case. Cases like the paradigmatic case ought to be treated likewise; cases unlike the paradigm ought to be treated differently. Thus, a man is properly charged with premeditated murder if the circumstances surrounding his case closely resemble the exemplar premeditated murder case. The less a given case is like the paradigm, the weaker the justification is for treating that case like the paradigmatic case.
Casuistry takes a relentlessly practical approach to morality. Rather than using theories as starting points, casuistry begins with an examination of cases. By drawing parallels between paradigms, or so-called "pure cases", and the case at hand, a casuist tries to determine a moral response appropriate to a particular case.
Casuistry has been described as "theory modest". One of the strengths of casuistry is that it does not begin with, nor does it overemphasize, theoretical issues. It does not require practitioners to agree about ethical theories or evaluations before making policy. Instead, they can agree that certain paradigms should be treated in certain ways, and then agree on the similarities, the so-called warrants between a paradigm and the case at hand.
Since most people, and most cultures, substantially agree about most pure ethical situations, casuistry often creates ethical arguments that can persuade people of different ethnic, religious and philosophical beliefs to treat particular cases in the same ways. For this reason, casuistry is widely considered to be the basis for the English common law and its derivatives.
Casuistry is prone to abuses wherever the analogies between cases are false.