"Obey is a difficult concept for someone in your position. "

This is a very difficult concept for young children to understand.

It's really not that difficult a concept to wrap one's mind around.

It is, however, a difficult concept to understand and translate.

It was a difficult concept, but one that could warm the heart in the dark night of space.

"A difficult concept to deal with at five o'clock in the morning."

It's a difficult concept that takes several years to understand.

"Peace is a difficult concept for some people," Data put in.

Is this such a difficult concept for the federal government to grasp?

To begin with, censorship is a particularly difficult concept in law.