Based on the discussion under the "Socrates" thread and some philosophical observations by Phaedrus, I thought up a way of thinking of the functions in terms of their associated fundamental skepticisms and beliefs.
This list could probably use some refinement; I would be interested in people's comment.
Ti:
Skeptical of objective reality. Skeptical of absolutes, and of moral or aesthetic principles. Believes in the value of systems.
Ni:
Skeptical of systems. Believes in "ideal states." Skeptical of the ability to understand the physical world directly; the world of ideas is more real.
Te:
Skeptical of anything that can't be clearly demonstrated. Interested in debunking mystics and other "magical" thinkers. Believes in what can be demonstrated rationally.
Si:
Skeptical of the value of things other than physical pleasure or activities one enjoys (whether work or play). Believes in one's individual power to experience things in a way that will make everything all right.
Se:
Skeptical of what one can't see or experience personally; skeptical of theories. Believes in one's own experience.
Fi:
Skeptical of others' intentions. Believes in moral principles.
Fe:
Skeptical in the value of things that don't relate to people. Believes that certain priorities and actions can make life meaningful and worthwhile.