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personality is a word we use to describe behavior

I think you might have it backwards,in a sense behavior is the word we use to describe personality isnt it? We behave according to our personality traits. Our personality, will, mind and emotions drive our physical bodies, not the other way around.

As fastlane will surely confirm, the physical universe is operating under the influence of physical law such as gravity. Why does gravity work? Science doesnt understand why gravity works, it only measures the behavior of the phenomenon.

Science is the perfect way to analyze how things work, science only fails when it trys to understand why things work. That is the precise reason why idiotic theories like evolution were born or ridiculous parallel universes; because science is trying to figure out why we are here instead of simply observing the world around us(which is its true function)