Type 3, however, can be uniquely identified, since 
  
    
      
        X
      
    
    {\displaystyle X}
   and 
  
    
      
        Z
      
    
    {\displaystyle Z}
   are marginally independent and all other pairs are dependent. Thus, while the skeletons (the graphs stripped of arrows) of these three triplets are identical, the directionality of the arrows is partially identifiable. The same distinction applies when 
  
    
      
        X
      
    
    {\displaystyle X}
   and 
  
    
      
        Z
      
    
    {\displaystyle Z}
   have common ancestors, except that one must first condition on those ancestors.
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (a cause) contributes to the production of another event, process, state, or object (an effect) where the cause is partly responsible for the effect, and the effect is partly dependent on the cause. In general, a process has many causes, which are also said to be causal factors for it, and all lie in its past. An effect can in turn be a cause of, or causal factor for, many other effects, which all lie in its future. Some writers have held that causality is metaphysically prior to notions of time and space.Causality is an abstraction that indicates how the world progresses. As such a basic concept, it is more apt as an explanation of other concepts of progression than as something to be explained by others more basic. The concept is like those of agency and efficacy. For this reason, a leap of intuition may be needed to grasp it. Accordingly, causality is implicit in the logic and structure of ordinary language.In English studies of Aristotelian philosophy, the word "cause" is used as a specialized technical term, the translation of Aristotles term αἰτία, by which Aristotle meant "explanation" or "answer to a why question". Aristotle categorized the four types of answers as material, formal, efficient, and final "causes".