For Aristotle and Euclid to appear from the start among elements of basic education is not so normal (5)
I believe the answer is:
rarer
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for rarer that I've seen before include "More unusual" , "More scarce or uncommon" , "Stranger" , "Less frequent" , "even less common" .)