The experience is well known which Augustine describes in the 10th Book of his Confessions, when he says that he understands precisely what time is when he does not reflect upon it. But as soon as he directs his attention to it and wants to say clearly what time really is, then he is completely non-plussed. This famous description of the problem of time in the introduction of Augustine's analysis appears to me to be the prototype of all genuine philosophical perplexity. Self-concealment in the thoughtlessness of what is self-evident is like a great resistance, which is unconquerable because of its lack of that with which philosophical thought and its desire to comprehend constantly have to contend.