Premise is the moral/ethical question defined (loosely) before play, posed during play.

• How far will you go to get what you want?
• What would you give up for Knowledge and Power?
• Is the life of a friend worth the safety of a community?
• Hundreds of others...

By defining Humanity in such a way as to tie into this premise (or by determining the premise by the definition of Humanity... it can go in either order), you set up a situation where the players' Humanity-relevant choices in game (often related to Bangs) will give you that particular character's answer to that Premise-question.

It is the collective answering of the Premise that creates the Theme of the story.

Sorcerer, by its very structure, is all about why people compromise their morals, and what happens to them once they do so. For very compelling reasons, the sorcerer has made a bargain with the (metaphorical) devil. While sorcery might achieve your goals, your Price is a constant reminder of what you've given up. And as the pressure mounts, your new friends aren't as trustworthy as you once thought, there's danger all around, and you're far from the person you'd hoped to be. What happens to people like that? Play the game and find out.

Example:

In a game, the premise is defined as "How far will you go to get what you want?" Humanity is defined as Empathy -- the emotional/visceral understanding and connection to others.

In a scene, one character's demon goes into Need (which in this case is Suffering). There are many ways the character could feed that need, but he is in a hurry and chooses to first search out a place with a lot of human suffering and then act in such a way that the Suffering already extant in the situation is also increased so that the need is satisfied more quickly.

In using his fellow man for his own ends, without regards to the effect on them (i.e.: In direct rejection of this game's definition of Humanity.) the player has, though play of his character, created an answer to the Premise ("How far will you go?"), and his actions create a Theme of the game.