John Playfair (1748–1819) was a Scottish mathematician and geologist best known for his defence of James Hutton's geological theories. He attended the University of St Andrews, completing his theological studies in 1770. In 1785 he was appointed joint Professor of Mathematics at the University of Edinburgh, and in 1805 he was elected Professor of Natural Philosophy. This highly influential book, first published in 1802, contains Playfair's clarification and summary of Hutton's geological concepts. Playfair concisely explains Hutton's theories on erosion and geothermal heat in rock formation and the concept of uniformitarianism in geology, illustrating these theories with his own precise observations on different types of rock strata. The clarity of Playfair's explanations was instrumental in popularising Hutton's geological theories, many of which are now recognised as key principles of modern geology. Playfair's strident defence of Hutton's ideas formed part of a controversial debate between Hutton's supporters and his detractors.