Te and Fi both consider the existance of a thing in itself to be a truth, and subjugate expression to existance. Te sees things that actually exist in an external reality, data. Fi sees things that actually exist in an internal reality, emotions. They create rules, but only based on their observations of what exists.
Fe and Ti both consider rules aspects of reality, and subjugate reality and expression to rules. Ti tries to use rules to define/predict internally what exists (or should exist) in external reality, data. Fe tries to use rules to define/predict externally what exists (or should exist) in internal reality, emotions.
Therefore, Te and Fi are in fact the same process engaged in reverse, as are Fe and Ti. This is why you always see those processes together in the inital four functions.
This means that people with Te and Fi are focused on describing what's there as an assumed truth, and people with Ti and Fe attempt to define what should be there using rules and principles. Does this make any sense so far?
Now, you may think I've mistaken Te and Fi for perceiving functions, but I haven't. Their function is still to evaluate things in terms of rules, they just base these rules on what they actually perceive within their respective forms of reality, rather than seeing rules an independant entities to which one can subjugate reality as Ti and Fe do. So, Te/Fi subject their rules to reality, and Ti/Fe subject their reality to rules.
The perceptions themselves, however, are separate, and simply feed the above processes.
Se and Ni both deal with reality as it happens, believe it or not. Se simply appears to look outwardly at reality as it happens, and unconsciously is reacting to an abstract model of it created by the senses and mind, believing it to be reality. In other words, Ni works unconsicously, creating a representation of reality. Because we only preceive things in our minds, through our senses, we cannot see reality, only a representation of it. Whoever said, "We do not see things as they are, we see things as we are" was quite correct.
Ni is a bit different. It focuses on the creation of the abstaction itself, aware that it is only creating a representation of reality. With the awareness of your mind constructing this abstraction, you have to consciously focus on creating it. But with this awareness, you aren't limited to representations of reality. You can take an awareness of any given thing, and represent it internally. Se works unconsciously, feeding data from the senses with which you consciously try to create the abstractions. In other words, Se percieves the "reality", Ni perceives the internal construction of the "reality".
Si and Ne both actually try to fit reality into patterns. Si simply appears to compare the reality that exists to reality that has previously been experienced. But truthfully, it is actually comparing the abstraction just unconsciously created to represent "reality" with a previous set of abstractions that it believed did the same. Ne works unconsciously, perceiving obvious patterns via which to compare previous abstractions with the new one, and the overlapping and most common trends in these result in "standards" by which reality is judged, as well as memories of how one situation was, based on how it differed from another.
Ne is different, again. It focuses on the process of seeking the patterns in the data itself, aware that it seeks patterns and not previous experiences. However, with the awareness that the patterns are being sought, it can look for patterns in several ways, rather than simply through previous experiences and information. It can proactively see patterns between things that are around it, things it has experenced before, and things that it has envisioned. It constantly tries to create new patterns and new associations. Si works unconsciously, holding up previous memories and associations from which patterns can be detected. Without an awareness of previous data, there would nothing from which to deduce patterns.
So, to recap, this means there are four two-sided functions:
Ti/Fe -- Creates rules by which to subjugate reality.
Fi/Te -- Creates rules which are determined by reality.
Se/Ni -- Deals with the experience and construction of our reality.
Si/Ne -- Deals with the comparison of reality to other things.