Put simply, the Perceiving* functions can be thought of as "What is", and the Judging functions as "What ought to be". But why is this? After all, both Judging and Perceiving IEs are simple combinations of Internal/External, Static/Dynamic, and Objects/Fields. So why are Ni and Si paired with Ne and Se? What is it about that specific combination that can be called "What is", and what is it about Ti, Fi, Te, and Fe that can be considered "What ought to be"?
After much thought, I have come to the following simple conclusion:
Perceiving consists of Statics of Objects and Dynamics of Fields. For Perceiving, the objects out in the real world are fixed and unchangeable, and the connections and relationships between them are malleable. This leads to a feeling of "I'm exploring the way the world really is, and changing how I relate to it accordingly."
Judging consists of Dynamics of Objects and Statics of Fields. For Judging, it's the connections between things that are fixed and unchangeable; the objects in the "real world" are what can be changed and manipulated. This leads to a feeling of "I know how things should relate to one another, and now I'm going to change the way the world is accordingly."
Thoughts? Questions? Criticisms?
[*Note: I'm using the terms "Perceiving" and "Judging" here instead of "Irrational" and "Rational", because the former describe the specific difference I'm talking about more clearly in this case.]