Pafnuty Chebyshev (1821–1894) was a Russian mathematician, the founding father of a whole school of Russian maths, and a man who genuinely believed that pure theory and greasy machines belonged together. He built calculating devices, studied the maths of walking, and taught the teachers of a generation of legends.
Chebyshev's inequality is a famously reassuring result: it guarantees that data can't stray
too far from its average too often, no matter what shape the distribution has. That single
bound is the workhorse behind proofs about how sequences of random quantities settle down —
the study of
Chebyshev loved mechanisms. He designed dozens of linkages — clever arrangements of rods and hinges — including the "Chebyshev linkage," which converts spinning motion into an almost straight line, and a plantigrade machine that walks. He treated engineering problems as serious mathematics and mathematics as something that should build real things. He also founded a dynasty of students, including Markov and Lyapunov, whose work still fills modern probability textbooks.