First, that sound is everywhere in spoken English (10% of speech) ...

... and it’s the simplest sound ... and it creeps part of the way into how we want to describe everything else in spoken English ... and things get defined with respect to that sound ... and it’s real (we’re not talking about the symbol ǝ but the sound itself) ...

If it’s real, and pervasive, and everything else in the domain gets defined in terms of it ... then it fits the bill for being a concrete universal.

The schwa sound is the best starting point, and vantage point, for researching and understanding how we use spoken English.

So, what’s so special about the sound of schwa?