For Ruskin the study of buildings was inseparable from the study of nature. He held that all true ‘forms and thoughts’ in architecture were ‘directly taken from natural objects’ (Works 8.141). In this lecture, Christopher Donaldson shall examine the origins of this ideal and trace its development over the course of Ruskin’s works, from his boyhood studies of Lake District cottages to his lectures at Oxford in the 1880s. This body of work is diverse, but it is underpinned by a conviction that architecture is both ‘a science of the rule and compass’ and ‘a science of feeling’ (Works 1.5). Early in his career, Ruskin referred to the study of this dual science as the ‘Poetry of Architecture’, and the speaker shall consider the aptness of this expression to many of his later assertions about building and design. More than just providing a survey of Ruskin’s writings, however, Christopher Donaldson's aim in this lecture will also be to explore the significance of his thinking about architecture for the 21st century. To that end,the speaker shall conclude by considering the new relevance Ruskin’s principles have found in our environmentally anxious age.