Welcome to Gaia! ::

Common sense and ethics are simply mathematical logic given a slightly new form and used more commonly on a day to day basis. Common sense is the practical application of logic, though not in the sense that one runs their day off what conclusion whatever proof they had written gives them, but in that there should be an obvious flow to the reasoning one has to do what one does. I.E.

1. I am hungry, ergo
2. I find food
3. The food is raw and thus unfit for eating, thus I
4. Cook it

Ethics is a bit different than this, thought that is a wholly different discussion for another time and it has no place in this conversation.

As far as you example goes, the logic would be true if there wasn't the underlying factor of mental illness. While it is true that the dead require no food, this man is not dead. He may perceive himself to be dead, but he is not. As much as he may think he is dead his belief in such is not enough to impose upon him the physical condition of death. Understandably if left alone he may simply not eat and eventually starve, but he was not dead when he first began to assume he was dead(at the onset of his illness.) In short, he may have had solid logic but the information he used was clearly false to lead to his justification, thus his argument was valid but his information was false thus making his argument unsound.

Edit: And I shall be right back. Packing up my things and heading to a friends house. I shall respond to any questions you may have Lu when I return. 3nodding