During a 'debate' with a friend about same sex marriage, he raised the issue of marriage being 'by definition union between a man and a woman', and appeared to hold that this was grounds for rejecting same sex marriage. My question does not relate to the ethics surrounding the issue, but rather to the fallacy I thought he had commited in saying this. It seemed to me as if he was stating the conclusion of an argument that had not been argued (at least, not by us either at or prior to that time) namely whether marriage is, in fact, the union as mentioned - is this what is known as 'begging the question' (i.e., stating a point that remains to be proven as foundation for another conclusion)? If not, then what is the formal term for this fallacy (if it is, indeed, fallacious)?

The argument from definition was actually put forward in the Canadian courts a few years ago, when they were debating the question of whether gay marriage should legalised. It was argued (roughly speaking) that it makes no sense to try to legalise gay marriage, because, by definition, marriage is a union between people of different sexes. So - it was argued - trying to lagalise gay marriage would be like trying to legalise married bachelors or male vixens. The argument was rejected. One unsubtle problem with the argument is that the premise seems to be false. Here is a quote from the Random House Dictionary (1983 edition): "Marriage: a relationship in which two people have pledged themselves to each other in the manner of a husband and wife, without legal sanction: trial marriage, homosexual marriage" It seems to me that this is a common occurrence: people fairly often try to argue for some conclusion or other by appealing to the definition of some key term, when they actually don...