The perceptibility of things is not the end of their being. Their surface is available to our tools, their depth is immune to our inquisitiveness.
Things are both available and immune. We penetrate their physical givenness, we cannot intuit their secret. We measure what they exhibit, we know how they function, but we also know that we do not know what they are, what they stand for, what they imply.

Ch. 5. - Who Is Man? (1965)