(CNN)Let's get one thing out of the way: Hateful, nasty, vitriolic speech, even when it's bigoted or homophobic, is often protected.
Quiz: Is hate speech free speech?
There's been a lot of talk since the Portland, Oregon, train stabbings about hate speech and whether it is or should be legal. The short answer is it's legal, though there are rules against threatening people and inciting violence. There are also some nebulous court rulings about "fighting words" and obscenity.
But for the most part, telling people to go back to their country or that their race is inferior or that they're less human because they are gay, Latino, female, whatever -- that's legal, not that it should be encouraged in civil society.
Whether it should be legal is another question, and while we won't wander too far down that path, it's important to remember the US Supreme Court has repeatedly ruled that it's the most offensive idea -- and not the notion with which everyone generally agrees -- that deserves protection.
In the words of Justice William Brennan in 1989: "A principal function of free speech under our system of government is to invite dispute. It may indeed best serve its high purpose when it induces a condition of unrest, creates dissatisfaction with conditions as they are, or even stirs people to anger."
Added Chief Justice John Roberts 22 years later: "Speech is powerful. It can stir people to action, move them to tears of both joy and sorrow, and ... inflict great pain. On the facts before us, we cannot react to that pain by punishing the speaker. As a Nation we have chosen a different course -- to protect even hurtful speech on public issues to ensure that we do not stifle public debate."
Think you understand what's protected? Take our quiz and see how well you know your speech: