it seeming easier to make one’s self invisible to others, than to make another’s thoughts visible to me, which are not visible to himself.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“and yet can see that dogs or elephants do not think, when they give all the demonstration of it imaginable, except only telling us that they do so. This some may suspect to be a step beyond the Rosicrucians; it seeming easier to make one’s self invisible to others, than to make another’s thoughts visible to me, which are not visible to himself. But it is but defining the soul to be “a substance that always thinks,” and the business is done. If such definition be of any authority, I know not what it can serve for but to make many men suspect that they have no souls at all;” source