John Walker Gray died on May 17th, 2017 aged 85. It was not unexpected.

He died as he lived, in a comfortable and well-planned manner, his needs anticipated, and care taken that his affairs be in order. He was there for my first breath and I was there for his last, as is good and right. His foresight gave him, at the end of his time, an interval of rest and contentment that all deserve, but few are afforded.

Born to a merchant, he imagined for himself a life of the mind. Through hard work and patient dedication, he made that a reality, becoming an erudite and well-traveled man, a talented pianist, and a mathematician of note in several fields. The arc of his life was a work of his own devising, and it was good.

John went from an undergraduate degree from Swarthmore College in 1953 to his PhD from Stanford University in 1957 as a student of the renowned Charles Loewner. His PhD thesis on contact structures, published in the Annals of Mathematics, remains of interest today, having been cited nearly a hundred times in recent years alone.

As a Professor of Mathematics at the University of Illinois, John supervised five PhD students, and at the end of his career served as Director of Graduate Studies. His research interests centered on Category Theory, a study of patterns common to many parts of mathematics including algebra, topology, logic, and computer science. His 1974 book on Formal Category Theory has long been a standard reference. John contributed to categorical semantics of programming languages including versions of the lambda calculus, and he attracted several of the founders and early employees of Wolfram Research to Urbana. He made his mark, advanced the art, and his students take a part of him into the future.

John was preceded in death by his wife of many years, Eva Wirth Gray, who introduced him to Switzerlanda