Why are there Only Onions on an emptu skewer? We are not saying that the empty skewer contains an onion. The key is the word _only_ and we are saying that all those things on the skewer are only onions. So if you have OOOO-skewer it's obvious: are any of them not onions? No -- so it's true. For OTO-skewer it's obviously false: Are any of them not onions? Yes, so it's false. For an empty skewer we need to check whether any of them are not onions? No, so the answer is false.
Now the core of your question concerns the following logical question,
which comes up in many different situations, not just here:
- if you have an empty collection of men, are all of them bald?
- if you have an empty collection of numbers, are all of them even?
- if you have an empty skewer, are all vegetables on it onions?
The _logical_ convention is to say "yes" in all three answers, with the justification that every single one of the elements in the set satisfies the condition, i.e. for all elements, the presence of an element in the collection implies that statement "is bald", "is even", or "is onion".
Why is there no 5th Bit of Advice? We decided not to include a Bit of Advice in this chapter, so there is no 5th Bit.