The philosophical rationalists believed that there was a priori knowledge about the world, or general truths about the world known by the mind, without experience. This was in contrast to the empiricist insistence that all of our knowledge about the world was based on experience, sensory information in particular. The seventeenth century philosophical rationalists, such as René Descartes (1596–1650), were opposed to the intellectual methods of the empiricists, but they still took science into account in their philosophies. Descartes was actively involved in scientific exploration and experimentation throughout his philosophical career. In the late-eighteenth century, David Hume’s (1711–1776) empiricism posed a special problem for Immanuel Kant (1724–1804) because Hume (1711–1776) applied skepticism to basic beliefs that many had taken for granted before him, such as the existence of God and the powers of natural causes to bring about their effects. In the nineteenth century, modern reactions against empiricism took hold in the work of Georg Wilhelm Friedrich Hegel (1770–1831), Friedrich Nietzsche (1844–1900), and early existentialist philosophers, such as Søren Kierkegaard (1813–1855). These reactions shared a concern for the validity of a priori truths and religious knowledge.