That is to say, it would make good sense grammatically to say either "A is the cause and B the effect" or "B is the cause and A the effect", though only one of those two can be actually true. In this view, one opinion, proposed as a metaphysical principle in process philosophy, is that every cause and every effect is respectively some process, event, becoming, or happening. An example is his tripping over the step was the cause, and his breaking his ankle the effect. Another view is that causes and effects are states of affairs, with the exact natures of those entities being less restrictively defined than in process philosophy.Another viewpoint on the question is the more classical one, that a cause and its effect can be of different kinds of entity. For example, in Aristotles efficient causal explanation, an action can be a cause while an enduring object is its effect. For example, the generative actions of his parents can be regarded as the efficient cause, with Socrates being the effect, Socrates being regarded as an enduring object, in philosophical tradition called a substance, as distinct from an action. Epistemology
Since causality is a subtle metaphysical notion, considerable intellectual effort, along with exhibition of evidence, is needed to establish knowledge of it in particular empirical circumstances. According to David Hume, the human mind is unable to perceive causal relations directly. On this ground, the scholar distinguished between the regularity view on causality and the counterfactual notion.
If there is reason to believe that none of the 
  
    
      
        
          x
          
            j
          
        
      
    
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  s is caused by y, then estimates of the coefficients 
  
    
      
        
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   are obtained.