Many people are familiar with the Black-Scholes and Merton model for valuing options but not many people seem to be familiar with Stephen Ross’ generalization. His model can handle any collection of instruments, not just a bond, stock, and European option.

“A Simple Approach to the Valuation of Risky Streams” https://www.jstor.org/stable/2352277

Maybe Ross blew people’s minds when he then used it to “unify a number of topics in financial economics, including project valuation, Modigliani-Miller theory, forward pricing, the closed-end mutual fund paradox, and efficient market theories.”

It does not require Ito calculus or partial differential equations. Although positive measures of mass 1 show up, it is a categorical error to assume they are the probability of anything. Every model is just a stochastic process. Instead choosing a particular parameterization then fitting market data, why not use the market data directly?