Primary Axis: Introverted Thinking - Extraverted Intuition
The INTP is above all a thinker and his inner (private) world is a place governed by a strong sense of logical structure. Every experience is to be rigorously analysed, the task of the INTP's mind is to fit each encountered idea or experience into a larger structure defined by logic. For here is the central goal of the INTP: to understand and seek truth. The experience of anything takes a back seat. The INTP is not interested in experiences themselves but is far more fascinated by concepts. The drive to understand things that are not yet understood is a very powerful force in the life of an INTP. Where the Ti preference is strong, this drive can override the experiential element so strongly that the INTP will become quickly bored with anything that he has successfully analysed to the point of understanding it. Once understood, it has nothing left to offer, once the satisfaction which comes with achieving the goal of understanding diminishes. Indeed, most primary interests of an INTP are things which he cannot fully understand, usually because they are highly complex or have some exotic, mystical element that does not yield to analysis. This is the real reason why INTPs are drawn to complexity: anything simple is too quickly understood and cannot hold the fascination for long. Similarly, proficiency in any area (which requires continual practice after understanding) is not such a driving force as it might be for NTJs, for example. While a judging NT will often seek to become master of his field, an INTP is satisfied by analysing it alone. The latter is often more of dabbler with ideas which leads me on to his second crucial aspect: detachment.
The Ti-Ne axis leads to a curious overriding desire to observe from a detached position, indicating the preference for intuitive perception with respect to external things. Since accurate analysis needs to avoid becoming hampered with details or being influenced by the actions of others, the INTP invariably seeks to withdraw, at least in spirit, from the situation being considered. This detachment can sometimes be so marked that he will readily see himself as a neutral observer having no personal association with that going on around him (unless forced to become directly involved through an attack on his principles). The INTP enjoys speculating about how a news item (for example) might be received by other people or how a point of view might offend certain types of people and be supported by yet other types; but to have a point of view of his own rarely seems relevant! This also indicates that Feeling is his least developed function. The argument may even be made that "points of view" and "opinions" are irrelevant since only objective truth counts. In reality, INTPs can often become far less objective than they think they ought to be: precisely at those times when the under-developed Feeling gnaws at his being.
source - http://www.intp.org/intprofile.html