John Andrew Leslie is a Canadian philosopher who focuses on explaining existence. He was educated at Wadham College, Oxford, earning his BA in English Literature and his MLitt in Philosophy. He is currently University Professor Emeritus at the University of Guelph, in Ontario, Canada, and a Fellow of the Royal Society of Canada.

He is the author of Universes (the first comprehensive philosophical study of multiple universes and their implications); The End of the World: The Science and Ethics of Human Extinction; Infinite Minds; and Immortality Defended; and he is the editor of Modern Philosophy and Cosmology and co-editor, with Robert Lawrence Kuhn, of The Mystery of Existence: Why Is There Anything At All?, the first major edited volume (some fifty authors reprinted, going back to Plato and ending with philosophers, theologians and scientists of today) to tackle all
the main ways of answering its question. Oxford-educated, he served on the faculty of the Philosophy Department at the University of Guelph, Ontario, and in 1998 was the British Academy Royal Society of Canada Exchange Lecturer. His research interests center around the following categories: Metaphysics, with emphasis on the nature of causation, of time, of the mind, and of the world's unity, Philosophy of Religion, emphasizing the neoplatonist idea that God, instead of being an omnipotent creator, is a need for the world to exist, a need which is itself creatively effective and the Spinozistic notion that the world is just the thoughts of a divine mind, Philosophy of Cosmology, particularly the suggestion that there exist many universes, ours being one of the rare ones in which intelligent living beings can evolve, Ethics, particularly the duty to prevent the extinction of the human race, and Analysis of Risks through probability theory and decision theory.