Ethos. Pathos. Logos. The three pillars of the rhetorical argument, one of the most ancient ideas we’ll be exposed to during our schooling, rivaled in ancientness only by other Greek ideas. Greeks have all the best ancient ideas it seems.
So what are these three pillars, and how are they used to construct arguments and convey ideas?
First, Ethos.
Ethos is the credibility and character of the speaker conveying the idea. No matter how convincing your message may be, how solid the logic or science behind it is, or how passionate you feel about what you’re saying, if you’re coming across as a total snooker-loopy crackpot then no one is going to take you seriously.

Pictured: Not Ethos
Ethos strikes me as one of those things that you either have or you don’t. Either you’re a credible source for what you’re saying, or you’re not and that perception is going to cling about you. And nowadays what with the internets and googles of the world your credibility is easily challenged. If you don’t have it, it’s going to become readily apparent to anyone who bothers to fact-check you or look up other, less than credible opinions you may have posted on Facebook.

I just heard of this new band called "The Doors." You heard them yet?
This is also affected by one’s reputation. It’s easier to be considered credible if you happen to be a leading expert in your field. Less so if you aren’t. I knew a girl who had a bachelor’s degree in psychology and felt that that empowered her to analyze everyone around her. It’s pretty common knowledge though, that you’re not considered to be learned enough to actually practice psychology until you’ve taken the time and effort to get a doctoral degree. Needless to say, she lost her credibility among that particular peer group pretty quickly.
Even if some of what she was saying may have been correct, her lack or credibility deeply affected the view of what she was saying. As a result it became harder for people to take her seriously in other subjects as well.
Pathos.
Pathos is the listener’s emotion and how well the speaker manages to engage them. This can be a tricky part to manage. Even if your argument is backed by one’s own credibility and has a logical grounding it’s still possible to lose your listeners by mismanaging their emotional response.
This s where knowing your listeners really comes into importance. If you know their values and beliefs and culture, then it will be far easier to tailor your message to evoke the emotional response you’re looking for than if you didn’t. To connect to your listeners you have to know them and know what they’ll respond to.
This is easily seen in our politics. The Republicans appeal to the values of their constituents, which are very different than the values of people who consider themselves Democrats. The values held by both groups are different and the similar emotional responses both audiences can give are linked to vastly differing ideas.
Another great example of using Pathos is that I know some of my target audience well enough to know that one simple move will totally endear the people reading this to me forever. Behold:

Concentrated Pathos ... Aww, who's a kitty?
Logos.

Pure cold-brewed logic
Logos is my favorite of the three pillars. It’s the one that gets to me the easiest, and that without I’ll discount what you’re saying to be the easiest.
Logos in a nutshell: Does it make sense?
And that’s really it. If what you’re telling me has no grounding in reality, I’m not going to listen to you. And that’s regardless of how hard you try to play to my emotions. Try being cornered by an anti-government 9/11 conspiracy theorist who’s insistent that the South is going to rise again and you’ll know how crippling a lack of Logos can be to an argument.
To me, especially in the modern age where knowledge flows like a rushing river, this is the most important pillar. This isn’t the Middle Ages or Ancient Greece. The average citizen is knowledgeable, literate, and informed. The average person in our country today, even the dumb ones, are stuffed to the brim with information that would make them scholars and philosophers in the ancient world. The more information that flows around and that we, as a whole, absorb, the most you’re going to have to have an argument that’s grounded in reality for someone to listen to you.