Right. The senses are always functioning, but there’s an unconscious process that happens before anything gets to the point of being the object of a conscious thought. That conscious attention moves from one thing to another from moment to moment.
Conscious thought and conceptual discrimination happen in the mental consciousness, which has more function than the five physical sense consciousnesses. It’s the place where higher functions of mind like knowing and wisdom take place. The five sense consciousnesses just notice and pass on sensory perceptions to the sixth consciousness, according to Sarvastivadins, at least. They don’t process the sensory information conceptually. E.g., the visual consciousness doesn’t recognize colors, but it provides the input that the mental consciousness uses to recognize things.
Dhammajoti summarizes some of these theoretical positions in his book Sarvastivada Abhidharma, which is available as a pdf.
I just recently saw a passage in the Chinese commentary to the Udana Varga when I looked up parallels for someone else. It interpreted a metaphor of traveling alone as referring to the mind giving attention to only one sense field at a time but moving from one place to another freely.
There apparently a controversy with Mahasamghikas over this point regarding the Buddha’s omniscience. They held that the Buddha could know all things in a single thought, but Sarvastivadins held that this was impossible. He could only know what he gave his attention. So, it wasn’t a universal doctrine. Does anyone know if there’s an equivalent position in Theravada Abhidhamma?