Ah! I like this definition of Fi. I think I get what you mean, I've often thought it in similar terms. Fi in socion is that function thanks to which we assess how much we like or not something, ie. attraction-repulsion, and that's the main spring behind our actions, socionics functions or not. You can call it desire or you can call it "to know what ticks us", it's not that different, and that's even why Fi is associated with morals, values, feelings, because the subject that uses Fi has a greater self-consciousness of what their internal world is and their values are the reflections of these internal thoughts. Every person has desires, every person gets ticked by something different, but some people are more in tune with their own desires, while others for example are more in tune with what comes from the outside, with the outside desires. Fi is the most self-centered function, at best this can become great self analysis, knowledge of one's inner mechanism.