Józef Rotblat was born on 4 November 1908 to a Polish-Jewish family in Warsaw, then part of the Russian-ruled Kingdom of Poland, better known as Congress Poland. He was one of seven children, two of whom died in infancy. His father, Zygmunt Rotblat, built up and ran a nationwide horse-drawn carriage business, owned land and bred horses. Józef's early years were spent in what was a prosperous household but circumstances changed at the outbreak of World War I. Borders were closed and the family's horses were requisitioned, leading to the failure of the business and poverty for their family. Despite having a religious background, by the age of ten, he doubted the existence of God, and later became an agnostic.