Photo by Pictorial Press Ltd / Alamy Stock Photo In a series of letters written between 1771 and 1773, Jean-Jacques Rousseau warned his friend, Madame Étienne Delessert, against the practice of horticulture. Commonly thought of as a philosopher, Rousseau’s abiding interests were music and, especially in the declining years of his life, botany. If the study of plants was respectable and salutary, the landscaping of gardens, he thought, where the imprint of the gardener’s hand deforms nature, was a grotesque enterprise performed by monsters. Tutoring Mme Delessert on how to observe spring flowers, the philosopher-botanist wrote: To the extent that...