Complex sentence: The three stress tensors,\\ sigma, P and S, studied here were all found to be objective therefore they are all suitable to describe the material response and develop constitutive laws because they are independent of the observer.
Split into simpler sentences: The three stress tensors,\\ sigma, P and S, studied here were all found to be objective. Therefore, they are all suitable to describe the material response and develop constitutive laws, because they are independent of the observer.