I frequently come across the saying, "A foolish consistency is the hobgoblin of little minds." As a debate-stopper, it's right up there with Whitman's wheeze from "Full of Myself"1:
Do I contradict myself? Very well then I contradict myself, (I am large, I contain multitudes.)
Each is, for the most part, simply a somewhat literate way of saying, "So what if my arguments are invalid?"
Not that I deny there is such a thing as "foolish consistency." Insisting on the same prudential actions in different situations with obvious and relevant distinctions could be described that way.
It seems to me, though, that, on the list of all the faults we humans are heir to, "consistency" of any kind wouldn't rank very high. On the contrary, I'd say foolish inconsistency is far more common.
This thought came to me after reading a comment elsewhere that I took strong exception to. I ran through a list of the direct logical corollaries of that statement, picked out a particularly bad one, and began to compose a reply along the lines of, "Oh, really? If you think that, then you must think this."
I stopped myself before sending it. When people say such things to me, they're often wrong, because "this" in no way follows from "that." Before I fired off my unanswerable answer, I wanted to make sure that the this I had picked really did follow as the night the day from the that the other fellow had asserted.
That's when it occurred to me: It doesn't matter whether the this followed from the that. I was dealing with a human being, and human beings are perfectly capable of holding, in fact quite likely to hold, contradictory positions. The fact that P implies Q by no means means the fact I hold P implies I hold Q. "If you think that, then you must think this" ain't so.
One consequence is that devastating replies aren't always so devastating. "If you're right, then there's nothing wrong with pitchforking babies!" may be logically true, but it can be countered by a foolishly inconsistent, "Please, I'm not saying it's okay to pitchfork babies."
And if devastating replies aren't always so devastating, then perhaps they become cheap. "If you think that, then you must think this" is used, not to advance the debate, but to stoke up your side. It doesn't matter whether that really does imply this; no one's mind is going to be changed anyway. What matters is that "this" is Really Bad. And we wind up dulled to the presence of such arguments, meaning that when we run into one where "that" really does imply "this," "this" is Really Bad, and we happen to hold "that," we dismiss it without a thought about what it might mean for us to hold "that."
1. Seriously, the thing is almost 16,000 words. That's not a song, that's an opera, and almost twice as long as Verdi's longest.