Because, even if I have the worst abdominal pain, the sum of the three angles of a triangle is still 180 degrees. Thus, human beings have the ability to transcend physical needs and behave rationally. Female nurse be strong be brave be humble be badass poster. In that sense, the mind is superior to the body. Our feet may be weak, our backs may be cuffed and our teeth may fall out, but 2 plus 2 will always equal 4, as long as we have reason. Reason does not become old and weak. Only the body is getting old. For Descartes, the essence of consciousness is thought.