“On the east the high mountain-chain of Zagros, penetrable only in one or two places, forms a barrier of the most marked character, and is beyond a doubt the natural limit for which we are looking.”

— 1867, George Rawlinson, The Five Great Monarchies of the Ancient Eastern World:

“A Boer may know you, but it will take you some time to know him, and when a certain stage in your acquaintance is reached, you may begin to wonder whether his real nature is penetrable at all.”

— 1900, Arthur M. Mann, The Boer in Peace and War:

“A capacity is cognitively penetrable in this sense if that capacity is affected by the subject's knowledge or ignorance of the domain.”

— 1996, Peter Carruthers, Peter K. Smith, Theories of Theories of Mind: