[housecarpenter]

The paper says psychological, not physiological.

It looks like what Codd means by "psychological features" are features that are (or are intended to) make it easier for users to express what they want to do in the language, as opposed to "logical features" which are what actually enable things to do. It's similar to the concept of "syntactic sugar", I guess. In a well-designed language, there should be a relatively small, theoretically well-understood core of logical features, and the psychological features should be straightforwardly expressible in terms of those logical features. This makes it easier to understand how the language works, which to me, does seem like an important thing for a language (it makes it easier to learn, easier to optimize, easier to add new features to without causing surprising interactions, etc.)

Codd's complaint is that SQL isn't like this, specifically with regard to the psychological feature of being able to nest queries. This was intended to be a more user-friendly alternative to using predicate logic. But instead of designing the nested queries feature as a syntactic sugar layer on top of an underlying predicate logic core, both features were implemented at the "core" level. The result is a hodgepodge language which isn't clearly based on either predicate logic or nested queries alone.