What is the nature of knowledge and truth? What influences our attitudes, and the way we think? What is the nature of the Universe and our place in it? Such questions have interested philosophers for thousands of years. Each of the five books in this set address these questions in their own way. Conditions of Knowing criticises the naiveté of contemporary epistemological and metaphysical attitudes and sets forth suggestions for improvements. The Philosophy of 'As If', published in 1924 looks at how we engage in false ideas and judgements and aims to create a more fruitful relationship between positivism and idealism. The set also includes Frederick Albert Lange's classic History of Materialism, first published in 1890, and republished in 1925.