The funny thing about quotes is that as time passes it becomes near impossible to find its original author regardless, the following quote is great:
"The mind, once expanded to the dimensions of bigger ideas, never returns to its original size."
“The mind, once stretched by a new idea, never returns to its original dimensions.”
"Man's mind, once stretched by a new idea, never regains its original dimensions."
“The mind that opens to a new idea never returns to its original size.”
– some sources attribute it to Oliver Wendell Holmes...or Albert Einstein...or Ralph Emerson