causality, in philosophy, the relationship between cause and effect. A distinction is often made between a cause that produces something new (e.g., a moth from a caterpillar) and one that produces a change in an existing substance (e.g., a statue from a piece of marble). Aristotle distinguished four causes—efficient, final, material, and formal—that may be illustrated by the following example: a statue is created by a sculptor (the efficient) who makes changes in marble (the material) in order to have a beautiful object (the final) with the characteristics of a statue (the formal). Later philosophers developed other classifications of causes, often duplicatory. The scientific conception that given circumstances under controlled conditions must inevitably produce standard results is generally accepted by philosophers. Systems vary, however, in the degree of emphasis that they place on the role of chance in changing a situation. David Hume argued that, in seeking to explain any object or event, we have evidence but no proof that its putative cause produced an effect on it. Immanuel Kant thought the idea of cause is a fundamental category of understanding and a necessary condition for experience; others argue a strictly mechanical theory of causality. The introduction of the uncertainty principle into modern physics has necessitated a modification of traditional concepts.