M I N D
This aspect is the source of creative ideas, planning, reason, logic, and analysis. Temperament theory refers to this aspect as the NT "Rational" temperament.
Where the other aspects address what to do, why to do it and the impulse to get it done, it is the Mind aspect that provides the knowledge of "how" to accomplish a thing. In one sense, the Mind aspect is the negative, reductionistic, analytical side of thinking; in another, it is the positive, holistic, creative side of thought. Both are required for understanding what works, what doesn't work, and why.
Of all the four aspects of personality, this is the aspect I express most naturally. Whether through inborn ability or as a result of learned and practiced skill, I am most comfortable dealing with all things on the basis of reason. My personal understanding of reality is not swayed by appeals to emotion or tradition, nor do I act for the sheer sake of "doing something." I hold the views I do because I have gone and gathered what relevant facts I could find, organized those facts into a logical model of reality, and formed impersonal conclusions based on that model. If those conclusions contradict some beliefs, if my internal model of the nature of reality is shown to be in erorr, then I change those beliefs--I correct the errors in my mental model of the world.
A classic INTP (in terms of Myers-Briggs type theory), I tend to enjoy classifying and categorizing knowledge, which I then use to propose models for the causes of things. In terms of Feeling, I have models of Personality. Where thinking needs an outlet for Action, I make a living using Computers to build systems. In terms of a Spiritual appreciation of order, I try to find better ways to Reasoning. And to indulge my enjoyment of logic in its purest form, I try to promote good Science.