> As used here, the term "psychological" refers to what is often called the human-factors aspects of a language. The term "logical" refers to the logical power

> As used here, the term "psychological" refers to what is often called the human-factors aspects of a language. The term "logical" refers to the logical power of a language, especially the power achievable without resorting to the usual programming tricks, such as iterative loops. It goes on to give an example of two queries which "one might reasonably expect" to be equivalent but give different results. (Personally, I disagree, but maybe I've been biased too much by SQL.)