André Weil, one of the founders of the Bourbaki Group, once said: “A mathematician is someone who has discovered at least one new theorem.” By this standard, Kiyosi Itô was not just a mathematician, but a giant among them. His work transformed the way we understand randomness. He introduced powerful ideas like the stochastic integral and stochastic differential equations, which later became the foundation of what we now call Itô calculus. These tools help scientists describe systems that change in unpredictable ways—like stock markets, weather patterns, or particles moving in fluids. In recognition of his groundbreaking work, Itô was awarded the Wolf Prize in Mathematics in 1987. The citation praised him for his deep contributions to both pure and applied probability, especially for creating a new kind of calculus designed for randomness. The story begins earlier, with Albert Einstein. In 1905, he proposed a mathematical model to explain Brownian motion—the seemingly random movement of tiny particles suspended in a liquid. Later, in the 1920s, Norbert Wiener and others showed that Einstein’s idea could be made rigorous using advanced mathematics. This led to what is now called the Wiener process, a mathematical model of random motion. However, there was a major challenge: how do you define an integral—a key tool in calculus—when the function itself behaves randomly? This is where Itô made his breakthrough. In the early 1940s, he rebuilt the idea of integration from the ground up for random systems. In 1942, he published his first formulation of the stochastic integral. At first, his work went largely unnoticed. Written originally in Japanese, it took years before the global mathematical community realized its importance. Reflecting on this, Itô once compared mathematics to music and art. He said that while anyone can enjoy a symphony by Wolfgang Amadeus Mozart or admire a grand structure like the Cologne Cathedral, mathematics is different. Its beauty is hidden in formulas and symbols, and only those who understand its language can truly “hear” its music. He believed his work was like a musical score—one that few could read at first. It took more than a decade before other mathematicians began to understand and build upon his ideas, turning his original “melody” into a rich and powerful theory. Today, Itô’s equations are widely used to describe systems that evolve with uncertainty over time. From physics to finance, his ideas continue to shape how we model the unpredictable world around us.