I reprinted the authors "first cut" narrative here for convenience.

First cut.
"Ns" What Jung called "iNtuitives". Keirsey liken them to "Martians." Abstract. Introspective. Those who look *primarily* through their *own* "minds eye."
"Ss" What Jung calling the aspect "Sensing" Keirsey liken them to "Earthlings" Concrete. Observant. Those who look *primarily* at the world by their "percepts", using what's out there.

I believe what he meant is obvious, because states what he meant. But you asked for my thoughts, so...

As an INTJ, I was often told when i was younger that I had book sense, but no common sense. The reality is that what was common to me, was not common to others, and I came across as a know-it-all (without common sense) until I learned to subdue my mouth and wield my gifts with a little more gracefulness. As N's, the ENFP's and INTJ's like to experience life primarily through our intuiting function, Ne (extraverted iNtuition) for the ENFP's and Ni (introverted iNtuition) for the INTJ's. For the ENFP's (and ENTP's), this is an interpreting function, and it means interpreting situations and relationships; picking up meanings and interconnections; being drawn to change “what is” for “what could possibly be”; noticing what is not said and threads of meaning emerging across multiple contexts. For the INTJ's (and INFJ's), this is a foreseeing function, and it means foreseeing implications and likely effects without external data; realizing “what will be”; conceptualizing new ways of seeing things; envisioning transformations; getting an image of profound meaning or far-reaching symbols. The remaining N's exhibit their Ne or Ni through their 2nd process, which is a supporting role; i.e., it is used after the primary process either fails, or it cannot or should not be used for some other reason.

I think the connection to "earthlings" for Sensors is that EVERYONE has the capabilitiy to see what is around them, what is there... but NOT EVERYONE has the ability to notice the NOT obvious, or to READ YOUR MIND (although I have met a few people who think they do... and some come close)... and us N's do like to see things or communicate things from our own, unique perspectives, don't we? Compound that with the fact that we are only 25% of the general population, and we are already fighting a losing battle against the Earthlings (fortunately, not the war, because we are smart enough not to fight our battles with the Earthlings 'head on').

By the way, out of ANY of the letters, the second one (N or S) is the ONE the causes the MOST problems with work place personality disagreements... it is what causes us to demonize, denograte, and detest our "other camp" coworkers.