The concrete universal mediates everything else in the domain. It defines the domain.

Language use is a big domain. It contains subdomains, such as the structure of syllables, prosody, the structure of words ...

Our concrete universal – the schwa sound – exists in all these subdomains and is a material place from which to explore the relationships between them.

We need a principled way of defining the domain we’re investigating – reading? ... speaking? ... listening? ... speaking and listening? ...

Otherwise it’s ad hoc to say what’s in or what’s out.