“All men naturally desire to know” : this is the celebrated assertion with which Aristotle begins the first book of his Metaphysics. According to him, human beings’ desire to know is as natural to them as their desires for food, rest, or amusement. These latter “natural” urges are responses to certain deficiencies—hunger, tiredness, and boredom; similarly, the desire to know is a response to a deficiency of knowledge. As Thomas Aquinas puts it, the urge to know (libido sciendi) is as much a characteristic of human beings as the urge to feel or to dominate (libido sentiendi and libido dominandi), and all three, essential though they may be to human nature, must be kept under control. My purpose here is to examine the limits of the desire to know: in what way may our desire to know go beyond the bounds of the “knowable”?