Logic is an important tool in debating, but it is very far from the whole story. There are other elements that are not logical but may carry the day, such as appeal to emotion, appeals to authority, precedent or tradition, empiricism, appeals to utilitarianism or aesthetics, personal attacks, aphorisms and violence. I believe they all have a place, except for aphorisms.
A couple of example “debates” to illustrate:
An engineer is proudly observing the bridge he has just completed. A second engineer talks to him:
2ND ENGINEER: How do you know your bridge is safe?
1ST ENGINEER: We calculated the maximum stresses and designed it based on the strength of the steel, with a safety factor of five. (Logical argument.)
2ND ENGINEER: How do you know a safety factor of five is enough?
1ST ENGINEER: It’s what the code stipulates. (Appeal to authority, which itself is based on empiricism and precedent.)
2ND ENGINEER: How did you calculate the maximum stresses?
1ST ENGINEER: We assumed nose-to-tail trucks carrying maximum permitted loads on both lanes.
2ND ENGINEER: What about nose-to-tail trucks on just one lane? Won’t that unbalance the bridge? (Proposing an addittional premise, implies current premises are insufficient.)
1ST ENGINEER: We took that into account.
2ND ENGINEER: What about force amplification by resonance?
1ST ENGINEER: We thought of that, the natural frequency is too high for it to happen.
2ND ENGINEER: What about a spill of burning gasoline?
1ST ENGINEER: It’ll take it.
2ND ENGINEER: A liquid oxygen spill that ignites the tarmac and steel itself?
1ST ENGINEER: (Becoming annoyed.) You’re not allowed to transport liquid oxygen by road.
2ND ENGINEER: Fuming nitric acid spill? Earthquake? Terrorist bomb? Plane crash? Meteorite impact?
1ST ENGINEER: Drops 2ND ENGINEER with one punch.
The second engineer has a valid point in that you can’t design for every contingency. The first engineer could have pointed out that a degree of pragmatism is required and you have to let some contingencies ride, but this is just as open to logical attack. Recognising that the 2nd engineer could continue forever and was deliberately being a pain, he opted for a succinct and practical communication of his feelings. Who won is itself a matter of debate; being a fan of utilitarianism myself, I’d give it to the first engineer.
My second example debate is between the 1st engineer and a philosopher:
PHILOSOPHER: How do you know your bridge is safe?
ENGINEER: We calculated the maximum stresses and designed it based on the strength of the steel, with a safety factor of five. (Logical argument again.)
PHILOSOPHER: How do you know that the strength of the steel won’t change suddenly? (Attacking a premise.)
ENGINEER: It has never been observed to change suddenly. Also, if materials changed strength suddenly and arbitrarily, it would have affected the development of the planets and the universe. There would be geological and astronomical evidence for it. (Appeal to precedent.)
PHILOSOPHER: That doesn’t prove that it couldn’t happen. Your assumption that the steel will maintain its properties over time is a matter of faith.
ENGINEER: True. I can’t prove that the universe will remain usefully consistent and predictable in the future, although it has done so up till now. On the other hand you seem to hold the same beliefs as me - I don’t see you clinging to the ground in case gravity suddenly reverses itself. (Personal attack, essentially accusing the philosopher of hypocrisy.)
PHILOSOPHER: Consistency is the hobgoblin of small minds! (Aphorism - an irritating and essentially invalid debating tactic, IMO, and probably worthy of another punch.)
ENGINEER: Whatever. Cross the river however you like. I’m going to use the bridge and take my chances with the universe. (Appeal to utilitarianism. What’s the quote about philosophers and toothache?)
Again, the philosopher is logically completely correct - the engineer’s belief in his calculations, the strengths of materials, the forces of gravity etc. are based on precedent, which doesn’t PROVE anything. His rebuttal is a pragmatic one rather than a logical one.
That’s enough rambling from me. I was going to start on about how Spock’s sacrifice at the end of Wrath of Khan wasn’t purely logical - his “the needs of the many outway the need of the few” has an implicit value premise based upon emotion, but I’ll leave it.