fundamentality as nonlinearity

In mathematics, the fundamental things are obvious. They are the axioms and the definitions. You play with them and the entire edifice changes. A single additional condition in your definition can cause a chain reaction resulting in a tremendous number of revisions in proofs that are dependent on your definition.

What is fundamental in product design is not that obvious. Features like Facebook's feed and Tinder's swiping unleashed an immense creative activity resulting in thousands of new analogical startups. Sometimes small UX changes like Snapchat's ephemerality can cause drastic changes in behaviour. 

In essence, what is fundamental can only be recognised when you nudge it. In other words, fundamentality is a perturbative notion: Greater the nonlinearity, greater the fundamentality. 

This interpretation works even in areas outside of mathematics, where there is no observable derivational depth. Large nonlinearities may be manifestations of aggregations of many small nonlinearities (as in mathematics and physics) or single "atomic" instances (as in social sciences where the human mind can short-circuit the observable causality diagrams).