We often find it hard to express our thoughts — to summarize our mental states or put our ideas into words. It is tempting to blame this on the ambiguity of words, but the problem is deeper than that.
Thoughts themselves are ambiguous!
At first, one might complain that that's impossible. I'm thinking exactly what I'm thinking; there's no way it could be otherwise. And this has nothing to do with whether I can express it precisely. But what you're thinking now is itself inherently ambiguous. If we interpret it to mean the states of all your agencies, that would include much that cannot be expressed simply because it is not accessible to your language-agency. A more modest interpretation of what you're thinking now would be a partial indication of the present states of some of your higher-level agencies. But the significance of any agency's state depends on how it is likely to affect the states of other agencies. This implies that in order to express your present state of mind, you have to partially anticipate what some of your agencies are about to do. Inevitably, by the time you've managed to express yourself, you're no longer in the state you were before; your thoughts were ambiguous to begin with, and you never did succeed in expressing them but merely replaced them with other thoughts.
This is not just a matter of words. The problem is that our states of mind are usually subject to change. The properties of physical things tend to persist when their contexts are changed — but the significance of a thought, idea, or partial state of mind depends upon which other thoughts are active at the time and upon what eventually emerges from the conflicts and negotiations among one's agencies. It is an illusion to assume a clear and absolute distinction between expressing and thinking, since expressing is itself an active process that involves simplifying and reconstituting a mental state by detaching it from the more diffuse and variable parts of its context.
The listener, too, must deal with ambiguity. You understand I wrote a note to my sister, despite the fact that the word note could mean a short letter or comment, a banknote, a musical sound, an observation, a distinction, or a notoriety. If all our separate words are ambiguous by themselves, why are sentences so clearly understood? Because the context of each separate word is sharpened by the other words, as well as by the context of the listener's recent past. We can tolerate the ambiguity of words because we are already so competent at coping with the ambiguity of thoughts.