Knowledge, traditionally, has been viewed from within, as something which we observe in ourselves rather than as something which we can see others displaying. When I say that it has been so viewed, I mean that this has been the practice of philosophers; in ordinary life, people have been more objective. In ordinary life, knowledge is something which can be tested by examinations, that is to say, it consists in a certain kind of response to a certain kind of stimulus. This objective way of viewing knowledge is, to my mind, much more fruitful than the way which has been customary in philosophy. I mean that, if we wish to give a definition of “knowing”, we ought to define it as a manner of reacting to the environment, not as involving something (a “state of mind”) which only the person who has the knowledge can observe. It is because I hold this view that I think it best to begin with Man and his environment, rather than with those matters in which the observer and the observed must be the same person. Knowing, as I view it, is a characteristic which may be displayed in our reactions to our environment; it is therefore necessary first of all to consider the nature of these reactions as they appear in science. Let us take some everyday situation. Suppose you are watching a race, and at the appropriate moment you say, “they’re off”. This exclamation is a reaction to the environment, and is taken to show knowledge if it is made at the same time as others make it. Now let us consider what has been really happening, according to science. The complication of what has happened is almost incredible. It may conveniently be divided into four stages: first, what happened in the outside world between the runners and your eyes; secondly, what happened in your body from your eyes to your brain; thirdly, what happened in your brain; fourthly, what happened in your body from your brain to the movements of your throat and tongue which constituted your exclamation. Of these four stages, the first belongs to physics, and is dealt with in the main by the theory of light; the second and fourth belong to physiology; the third, though it should theoretically also belong to physiology, belongs in fact rather to psychology, owing to our lack of knowledge as to the brain. The third stage embodies the results of experience and learning. It is responsible for the fact that you speak, which an animal would not do, and that you speak English, which a Frenchman would not do. This immensely complicated occurrence is, nevertheless, about the simplest example of knowledge that could possibly be given.