Concerning Conception and Articulation.

Again the reasoning part of the soul is divided into conception and articulation. Conception is an activity of the soul originating in the reason without resulting in utterance. Accordingly, often, even when we are silent we run through a whole speech in our minds, and hold discussions in our dreams. And it is this faculty chiefly which constitutes us all reasoning beings. For those who are dumb by birth or have lost their voice through some disease or injury, are just as much reasoning beings. But articulation by voice or in the different dialects requires energy: that is to say, the word is articulated by the tongue and mouth, and this is why it is named articulation. It is, indeed, the messenger of thought, and it is because of it that we are called speaking beings.

FideOrth 2.21.1