Calculus 8th Edition --- James Steward (Page 938) James Steward --- Page 930 Calculus 11th Edition ---- Ron Larson (Page 905) Both of the textbooks don't have the explanation of why we define Differentibility like this. If we use the definition then the proof of the chain rule is nice. But we need a more intuitive explanation about the definition. We'll get it in next article.