Social constructionists maintain that we invent the properties of the world rather than discover them. Is reality constructed by our own activity? Or, more provocatively, are "scientific facts" constructed? Is "everything" constructed? This is a discussion of the philosophical issues that arise out of this controversial debate, analysing the various strengths and weaknesses of a range of constructivist positions. The author argues that contemporary philosophical objections to constructivism are drastically inconclusive, while offering and developing objections. He shows that the strongest constructivist arguments still suffer from conceptual difficulties, illustrating the divide between the sociology and the philosophy of science through examples as varied as laboratory science, time, and criminality. Throughout, the book distinguishes between the social causes of scientific beliefs and the view that all ascertainable facts are constructed.