Explanation (or Proof) for network theorems?

[ArchKudo]

We started Network analysis and we were given Circuit Diagrams to be reduced using the following:

1. Kirchoff's Laws i.e KVL and KCL
2. Source Transformation
3. Superposition Theorem
4. Thevenin's Theorem
5. Norton's Theorem and
6. Star-Delta Conversion

However I wasn't given proofs for any of these theorem just their application (which is really easy) and now, I can't find their proofs on the internet too!
So, are these theorems based on empirical method or do they use higher mathematics?
And if not it would be great if someone could provide me with their proofs

ps: I can guess why some theorems are true like KVL or KCL because of energy conservation I suppose and Superposition theorem is relate-able to Superposition of electric fields and its relation with electric potentional but it is still confusing

 

[mfb]

They all can be derived from the Maxwell equations and charge conservation, those two are based on experiments. Introductory textbooks on electrostatics and circuits might cover that in more or less detail. Getting Kirchhoff's laws out of them is not so straight forward, getting the others from those laws is mainly mathematics.