Order the logical chain of special relativity.

1. Take it as given that every observer measures light at the same speed
2. Notice this is impossible if time and length are the same for everyone
3. Accept that measurements of time and length must depend on the observer's motion
4. Derive time dilation and length contraction as the consequences
5. Deduce that mass is itself a form of energy, E = mc squared

Everything follows from one postulate plus a refusal to tolerate contradiction. The famous mass-energy relation is a late consequence, not a starting point, which is why relativity is a good example of physics being deductive rather than a collection of separate discoveries.