What happens when we hear a sound? There are diffferent ways to answer that question. You might answer that something causes a vibration in air which travels as a pressure wave to the eardrum, where it is converted into a nerve impulse. That would be an explanation taken from physics, a theoretical answer, that is, a thought.
You might say that vibrations of the eardrum continue via the inner ear canal fluid and cause minute hairs to quiver which creates a nerve impulse which travels via synapses to a region of the brain dealing with sound. That’s a biological answer, again a theory – which doesn’t mean it is not correct – but nevertheless it’s a thought.
Neither of these options explain how a nerve impulse becomes consciousness of sound. What do we experience? We simply experience sound, directly. We don’t experience the object which caused it, although we may think of the sound as being that. Sound just happens in our experience.
Why is this important? Each of the three thoughts about sound travelling or being an object which caused the sound, includes the idea of two spatially distant ends of the process: an origin and a self which perceives the sound. The idea of duality comes with the thought. Probably we do not notice or question that. But the experience itself is not of two things, a self and a heard object, the actual experience is just of sound. It may well come with a sense of spaciousness inherent to the sound, but there is nothing of self in the sound.
How are we conscious of the notion of a self hearing, then? It comes as an assumption, one which fits with our ways of thinking (our theories and everyday habits), and then maybe we notice the physical sensation of a head, which also fits – but is not part of the sound-experience. So we do not notice the way our thinking brings us a sense of separation from the world, from objects, from experiences. Hey Presto – duality!
So we tend to “think our experience”, rather than notice the pure sensation itself. In meditation we can be simple and just be aware of the experience of sound, and see what effect that has upon our perception.