The answer is: Logos
In persuasion, Logos is a method to persuade people by using logic rather than emotion or ethical standard.
This means that persuasion made by logos will be objective, factually correct, and can be backed by scientific evidence.
In the example above, the friend suggested the other person to eat less and exercise more to lose weight. (no mention of feelings or moral). We can conclude persuasion is based on logic since in order to lose weight you need to burn more calories than you eat. This can be done by eating less and exercise more.
