Causality (also referred to as causation) is the relation between an event (the cause) and a second event (the effect), where the second event is understood as a physical consequence of the first.In common usage, causality is also the relation between a set of factors (causes) and a phenomenon (the effect). Anything that affects an effect is a factor of that effect. A direct factor is a factor that affects an effect directly, that is, without any intervening factors. (Intervening factors are sometimes called "intermediate factors".) The connection between a cause(s) and an effect in this way can also be referred to as a causal nexus.[citation needed]Causes and effects are typically related to changes, events, or processes; such causes are Aristotle's moving causes. The word 'cause' is also used to mean 'explanation' or 'answer to a why question', including Aristotle's material, final, and formal causes; then the 'cause' is the explanans while the 'effect' is the explanandum. In this case, there are various recognizable kinds of 'cause'; candidates include objects, processes, properties, variables, facts, and states of affairs; failure to recognize that different kinds of 'cause' are being considered can lead to debate.The philosophical treatment on the subject of causality extends over millennia. In the Western philosophical tradition, discussion stretches back at least to Aristotle, and the topic remains a staple in contemporary philosophy.