Randomness — Stochastic Models and Monte Carlo
Prices are random, so finance is fundamentally a probabilistic discipline — and the tools for reasoning about randomness are what let quants price the instruments that have no tidy formula. A stochastic model describes how a price wanders through time; Monte Carlo simulation lets you compute almost anything by running that model thousands of times and averaging. Together they are the computational engine of modern quantitative finance, and they're territory engineers find immediately familiar.
Prices are random, so finance is a probabilistic discipline. Brownian motion and geometric Brownian motion model how a price wanders; Monte Carlo simulation turns that model into a universal engine for pricing and risk — the computational core of modern quant finance, and territory engineers find familiar. (Educational, not financial advice.)