17. For should the enemy strengthen his van, he will weaken his rear; should he strengthen his rear, he will weaken his van; should he strengthen his left, he will weaken his right; should he strengthen his right, he will weaken his left. If he sends reinforcements every- where, he will everywhere be weak. 18. Numerical weakness comes from having to prepare against possible attacks; numerical strength, from com- pelling our adversary to make these preparations against us.