_C_ is a north pole and _B_ a south, as during their separation. If you proceed in a different fashion, this identity or similarity of parts will not be preserved; for you will perceive that if _C_, a north pole, be joined to _A_, a north pole, contrary to the demonstrated truth, and from these two lines a single one, _B A C D_, is formed, as _D_ was a south pole before the parts were united, it is then necessary that the other extremity should be a north pole, and as _B_ is a south pole, the identity of the parts of the former similarity is destroyed. If you make _B_ the south pole as it was before they united, then _D_ must become north, though it was south in the original stone; in this way neither the identity nor similarity of parts is preserved. It is becoming that when the two are united into one, they should bear the same likeness as the agent, otherwise nature would be called upon to do what is impossible. The same incongruity would occur if you were to join _B_ with _D_ so as to make the line _A B D C_, as is plain to any person who reflects a moment. Nature, therefore, aims at being and also at acting in the best manner possible; it selects the former motion and order rather than the second because the identity is better preserved. From all this it is evident why the north pole attracts the south and conversely, and also why the south pole does not attract the south pole and the north pole does not attract the north. CHAPTER X AN INQUIRY INTO THE CAUSE OF THE NATURAL VIRTUE OF THE LODESTONE