This article has as its focus the life and thinking of the practitioner and theorist Norman MacMunn (1877-1925), whose experimental work in a number of schools outlined a new conception of freedom and one that drew initially upon the thinking and practice of Maria Montessori. It explores how MacMunn used these new psychological ideas to develop innovatory practice, which included techniques he christened 'differentialism' and 'the partnership method'. The relationship of these to future educational developments is explored. The article also examines MacMunn's later work, which began to broaden his thinking into a wider theory of human relations, and which stemmed from taking charge of his own school as well as his disillusion with the First World War. It situates MacMunn within the broader development of progressivism and makes the case for him as being an important figure in the emergence of progressive thinking.