You promised me yesterday that one word could mean four different things. I've thought about it overnight and I still don't see how one thing can be four things.

A single word turns out to mean four different things at once — spoken, written, thought, and classified — taught by Sophia, resisted by Marcos, tested against the library itself, and precisely footnoted by Philo.

You promised me yesterday that one word could mean four different things. I've thought about it overnight and I still don't see how one thing can be four things.

Then let's find all four before midday. You won't have to imagine any of them — this room is full of all four.
The Word Spoken

Sophia! The eastern shipment has arrived!

There — the first sense, gone before you could hold it. A word spoken is only breath shaped into sound. The moment it's said, it's already over. Nothing left of it but what we remember.
The Word Written

But the clerk's shout is written nowhere. This scroll, though — shipment is written right here, in ink, and will still say the same thing tomorrow whether anyone reads it or not.

The second sense — a word written down. It outlasts the breath that first said it. That's the entire reason this building exists: to keep the spoken word from vanishing the way the clerk's shout just did.
The Word Thought

And if I think of a shipment, with my eyes shut, saying nothing and writing nothing — is that a word too?

The third, and the strangest — the word held only in the mind. Neither sound nor mark, and yet it's the one the other two are trying to carry. The spoken word and the written word are both just vessels for this one.
The Word Classified

That's three. You promised me four.

The fourth is the one that will occupy the rest of our year, Marcos. Ask what kind of word shipment is — not what it sounds like, or looks like, or means, but what job it does in a sentence — and you've asked the grammarian's question instead of the philosopher's.

Exactly right. A word can be sounded, written, thought, and classified — and grammar only ever concerns itself with the last of these. Every lesson from here on is really just: what job is this word doing?

Which means, before you can answer that question even once, you'll first need the whole list of jobs a word is permitted to hold.

A list. Of course it's a list.