The words they use—that is, the words they stir up or retrieve from our memory and presume to be their own—are well chosen, clear, meaningful, distinctly enunciated, and relevant. Surprising to say, the spirits are smarter and faster at choosing words than we are ourselves. As a matter of fact (and this has been demonstrated to me), they grasp a word’s various shades of meaning and apply them immediately, without any forethought. The reason, again, is that the thoughts composing their language flow only into suitable words.
The situation is almost the same as the times when we speak without thinking about the words and focus solely on the meaning of the words. Under those circumstances, our thought falls quickly and easily into phrasing that accords with our meaning. It is the inner meaning that produces the words. The same kind of inner meaning, but even more subtly nuanced and refined, is what the language of spirits consists in, and by it we communicate with them, although we are unaware of it.
from Secrets of Heaven, Volume 2, Section 1638