What is that which can be thought of only by those who have special thinking skills?
To answer this question I would say:
Ones own original thoughts.
In Socrates' story about the Egyptian God Toth, (father of letters,) Socrates says to Toth:
"You offer your pupils the appearance of wisdom, not true wisdom, for they will read many things without instruction and will therefore seem to know many things, when they are, for the most part, ignorant."