Let us begin by making a rather obvious remark: the meaning of the question of ‘what we do not know’ varies according to whether or not the word “yet” is explicitly or implicitly included. It comes as no surprise that it should be in physics, the science in which, ever since Galileo and Newton, the quest for knowledge has been so amply and unexpectedly rewarded, that we find the most dramatic examples of both possibilities: one in which theory points to knowledge not yet acquired, knowledge which is still to be conquered, but which, once attained, should constitute its final triumph; and one in which “we do not know” may be taken as the conclusion of established knowledge. We do not (yet) know how to unify the four fundamental forces of interaction in a single theory, and yet that eventuality is already being called the theory of everything (TOE). Since interaction is the principle on which physics now bases all its explanations, and since physical explanations are in principle valid for everything that exists, the unification of the forces of interaction must therefore be the science of the principles of everything that exists. On the other hand, we do not know at what moment a particular radioactive nucleus will disintegrate. In this case, our knowledge of physics makes it impossible to use the word “yet”: radioactive disintegration is described in terms of a “lifetime,” which means that each nucleus within a given population has, at any one moment, the same probability of disintegrating, and that this probability is not a function of any variable which we can identify and manipulate. In this case, our knowledge asserts that we cannot know now and will not be able to know for as long as our present knowledge remains valid.