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.
1900: Arthur M. Mann, The Boer in Peace and War
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.
1996: Peter Carruthers & Peter K. Smith, Theories of Theories of Mind
A capacity is cognitively penetrable in this sense if that capacity is affected by the subject's knowledge or ignorance of the domain.