"What enables man to know anything at all about the world around him? … Nothing can be known without there being an appropriate “instrument” in the makeup of the knower. This is the Great Truth of “adaequatio” (adequateness), which defines knowledge as adaequatio rei et intellectus — the understanding of the knower must be adequate to the thing to be known.[...]

For every one of us only those facts and phenomena “exist” for which we posses adaequatio, and as we are not entitled to assume that we are necessarily adequate to everything, at all times, and in whatever condition we may find ourselves, so we are not entitled to insist that something inaccessible to us has no existence at all and is nothing but a phantom of other people’s imaginations.[...]

There is nothing more difficult than to become critically aware of the presuppositions of one’s thought. Everything can be seen directly except the eye through which we see. Every thought can be scrutinized directly except the thought by which we scrutinize. A special effort, an effort of self-awareness, is needed: that almost impossible feat of thought recoiling upon itself — almost impossible but not quite. In fact, that is the power that makes man human and also capable of transcending his humanity."