During centuries, it was regarded as obvious that 'space', 'time' and 'matter' had an objective existence. Until 19th century, physicists tried to clutch to these 'ideas', although negative experiments accumulated.
Then, Einstein broke the taboo by establishing equivalence of 'matter' and energy, with his famous formula E=mc2. Furthermore, 'space' and 'time' disappeared for a combination of both, in agreement with the experimental results of the time (1905-1920).
Korzybski was strongly influenced by the considerable progress of physics at that time. He understood that this revolution was due to a profound change of premises and methodology.
One important lesson to draw from these new formulations is that it is very risky to separate verbally what cannot be separated empirically. It appears difficult to show any 'space', 'time' or 'matter', without showing the two other elements altogether.
Korzybski baptized "elementalist" (el) these words which separate what cannot be separated empirically. Conversely, he baptized "non-elementalist" (non-el) those which restored the union, broken at verbal levels. Thus "space-time", "psycho-somatic", "evaluation", "semantic reaction", etc., are non-el, whereas "space", "time", "body", "spirit", "feeling", "reasoning", "emotion", "logic", etc., are el.
The main tools of general semantics allowing to deal with this problem are the hyphen and the scare quotes.