Many of the fundamental ideas of computer science have been invented, explored and discussed by leading philosophers and logicians, long before computers were invented (by logicians, of course). This presentation by Tony Hoare, Microsoft Research, looks at the ideas of philosophers and logicians such as Aristotle, Euclid, St. Thomas Aquinas, William of Ockham, Leibnitz, George Boole, and of course Alan Turing, and explains their relevance to computing of the present day.