To respond to ideas means to respond to significant similarities in objects and also to significant differences. It means to note certain qualities that objects have in common, and to classify these common qualities and their consequences in the behavior of objects. To note similarities and differences in the behavior of objects is to enable individuals to act in the light of the future. The printing on this page would be to a dog or to a baby merely a blur. To the reader the black imprints are signs or symbols. To the animal a red lantern is a haze of light; to a locomotive engineer it is a sign to halt. To respond to ideas is thus to act in the light of a future. It makes possible acting in the light of the consequences that can be foreseen. Present objects or features of objects are responded to as signs of future or absent opportunities or dangers. Every time we read a letter, or act in response to something somebody has told us, we are responding not to physical stimuli as such, but to those stimuli as signs of other things. HUMAN BEINGS ALONE POSSESS LANGUAGE. The value of the period of infancy in the acquisition of habits and the unique ability of human beings to respond to ideas is inseparably connected with the fact that man alone possesses a language, both oral and written. That is to say, men alone have an instrument whereby to communicate to each other feelings, attitudes, ideas, information. To a very limited degree, of course, animals have vocal and gesture habits; specific cries of hunger, of sex desire, or distress. But they cannot, with their limited number of vocal mechanisms, possibly develop language habits, develop a system of sounds associated with definite actions and capable of controlling actions. Only human beings can produce even the simplest system of written symbols, by which visual stimuli become symbols of actions, objects, emotions, or ideas. Biologists--in particular the experimentalist, Watson--find, in the capacity for language, man's most important distinction from the brute. Language may be said, in fact, to be the most indispensable instrument of civilization. It is the means whereby the whole life of the past has been handed to us in the present. It is the means whereby we in turn record, preserve, and transmit our science, our industrial methods, our laws, our customs. If human relations were possible at all without a language, they would have to begin anew, without any cultural inheritance, in each generation. Education, the transmitter of the achievements of the mature generation to the one maturing, is dependent on this unique human capacity to make seen marks and heard sounds stand for other things. The extent to which civilization may advance is contingent upon the development of adequate language habits. And human beings have perfected a language sufficiently complicated to communicate in precise and permanent form their discoveries of the complex relations between things and between men.