 Partly as a result of serious overwork coupled with insomnia, Collingwood’s health went into decline from the early 1930s. In April 1931 he suffered complications arising from chicken pox and began to suffer from high blood pressure. He was granted leave of absence by the university; following his return, in the autumn of 1932, he began writing an important new book, regarded by many as the pinnacle of his philosophical achievement—An Essay on Philosophical Method (1933). This had its origins in the methodological introduction to the lectures on moral philosophy he had delivered annually throughout the preceding decade. The Essay was a sustained investigation of the nature of philosophical reasoning through an examination of the distinctive character of philosophical concepts. Following publication of the Essay, Collingwood focused his philosophical energies on the philosophy of history and the philosophy of nature. The lectures he delivered at this time later formed the basis of the posthumously published The Idea of History (1946) and The Idea of Nature (1945).