When a noise is heard, a child will identify its source. The source is often external. But as the child learns to speak, the source is not external. That is, when a word is spoken, it echoes in the ears and therefore the mind of the talker. Then in learning to read or speak silently, the external spoken word is internalized.
Silent words echo without an audible use of the vocal chords, if they are used at all.
They are heard in the mind. The infant or child needs to learn to recognize this silent voice as his or her own, a voice that may remain one and not be divided. Thought appears to involve the formation of images in the mind, the formation of words on the lips or in the mind, and the formation or the recall of sensation fragments from the physical senses of sight, sound, touch and heat, taste and smell. When awake, this thought should or may remain linked to everyday reality, the sometimes rude external environment. The thoughts of a person are not usually isolated.