Perhaps it’s one of the perils of Twitter (you can follow me at @EdWhelanEPPC), but I’m surprised how often I encounter folks who seem not to understand the role of a principle in argumentation.
To illustrate the point by a common example: A reader who objects to my defense of marriage asserts that marriage should be available to any two consenting adults who love each other. I respond by pointing out that the principle that the reader is (explicitly or implicitly) invoking would extend to adult incestuous relationships. The reader replies by taking offense that I’m supposedly “comparing” same-sex relationships to incest.
There are various potentially intelligent paths that the reader might instead explore for his reply. He might try to maintain his principle by alleging that genuine consent isn’t possible in incestuous relationships or, alternatively, by defending application of the principle to adult incestuous relationships. Or he might modify his principle: he might, for example, contend that incest presents special harms and posit that his principle applies except where those special harms are present. But what isn’t intelligent is to complain that I’m “comparing” same-sex relationships to incest.
(To be clear, I of course acknowledge that if a principle applies to two distinct phenomena, that means that those phenomena have the common feature of being covered by the principle—and thus are in that very limited sense “like” each other. But that’s plainly not what the objecting reader is taking, or feigning, offense at.)