I thought the phrase "First, define your terms" might have come from Plato but didn't know how to find out. Google to the rescue! Turns out it was from Voltaire, and actually went "If you would converse with me, define your terms."
Seems that we routinely insert "first" much the way we add "again" to that movie line from Casablanca, "Play it, Sam."
At any rate, a Wikipedia quote archive came up first in Google and I found it very interesting. It's informal even by Wiki standards, often in a question-and-answer collaborative process. Those who dislike the utter lack of centralized control endemic to Wikipedia should have nothing to complain about here, as it's obvious that no one is posing as an expert.
Not content to take this on faith, I Googled the phrase "Voltaire define terms" (without the quotation marks) and came immediately to quotations on http://www.thinkexist.com/. The relevant page not only confirmed the wording of the quote, but went on to list many other thought-provoking quotes from the French philosopher.
I had known he was responsible for saying he would defend to the death your right to say things he disagreed with, but hadn't thought about it in a long time. What I hadn't known was just how many wise and wonderful things he did say, and how well-collected and well-presented they are on this web site.
http://www.jstor.org/pss/420322 My Google search also yielded a good page from JSTOR, a scholarly repository I had visited before, but forget why. This article by Darius M. Rejali, a Reed College political scientist, is entitled Define Your terms! Dictionaries, Medievals, and Thinking About Concepts. Only the first page is available without logging in and I think that login is free, unlike some scholarly sites, but I didn't pursue it just now.
http://stackblog.wordpress.com/2007/01/19/define-your-terms-such-as-say-jesus/ Then there was this excellent blog post from John Stackhouse, a professor at Regent College, University of British Columbia. He explains that although most Canadians check the box for "Christian," most have an unclear picture at best of who Jesus was. This is discussed without religious fervor but in the academic sense, that in this instance as in any other, it makes no sense to try to discuss something without comprehension of principal salient points.
Caution: explicit language. My search also turned up this blog post at the other end of the spectrum, in a sense. Blogger Greta Christina observes that reports of a study on teenagers and oral sex fail to distinguish between fellatio and cunnilingus, and eventually decides that it is the former that is really being discussed but not the latter. This post elicited several interesting replies. The point is well-taken that without "defining terms," the study's conclusions are without much meaning.
All of this harks back to a question raised in my mind by the way we define words. The Meaning of Everything, as it described 70 years of writing for the first edition of the Oxford English Dictionary by volunteer conrtibutors, showed that it can be done, but that it requires consensus rather than submission to authority.
Which brings me full circle to the Wiki page on Voltaire's quote, and the following quote by John Locke added by one of the people responding to the initial question about defining terms:
The names of simple ideas are not capable of any definition; the names of all complex ideas are. It has not, that I know, been yet observed by anybody what words are, and what are not, capable of being defined; the want whereof is (as I am apt to think) not seldom the occasion of great wrangling and obscurity in men's discourses, whilst some demand definitions of terms that cannot be defined; and others think they ought not to rest satisfied in an explication made by a more general word, and its restriction, (or to speak in terms of art, by a genus and difference), when, even after such definition, made according to rule, those who hear it have often no more a clear conception of the meaning of the word than they had before.
Cited by Rowena Cherry from An Essay Concerning Human Understanding (1689) Book III, chapter 4. She mentions that she has studied both Voltaire and Locke. I now remember having read Locke and Paine and many others while at Duke University, and thinking at the time that Locke must be one of the smartest people who ever lived. There's nothing in the above quote to make me change my mind.