In short, the word is familiar both to science and to common sense, and is applied with equal propriety to the actions of physical objects and to the manners of men. The abstract sciences, quite as much as the concrete and descriptive, are equally concerned with behavior. "The chemist and the physicist often speak of the behavior of the atoms and the molecules, or of that of gas under changing conditions of temperature and pressure." The fact is that every science is everywhere seeking to describe and explain the movements, changes, and reactions, that is to say the behavior, of some portion of the world about us. Indeed, wherever we consciously set ourselves to observe and reflect upon the changes going on about us, it is always behavior that we are interested in. Science is simply a little more persistent in its curiosity and a little nicer and more exact in its observation than common sense. And this disposition to observe, to take a disinterested view of things, is, by the way, one of the characteristics of human nature which distinguishes it from the nature of all other animals. Since every science has to do with some form of behavior, the first question that arises is this: What do we mean by behavior in human beings as distinguished from that in other animals? What is there distinctive about the actions of human beings that marks them off and distinguishes them from the actions of animals and plants with which human beings have so much in common? The problem is the more difficult because, in some one or other of its aspects, human behavior involves processes which are characteristic of almost every form of nature. We sometimes speak, for example, of the human machine. Indeed, from one point of view human beings may be regarded as psycho-physical mechanisms for carrying on the vital processes of nutrition, reproduction, and movement. The human body is, in fact, an immensely complicated machine, whose operations involve an enormous number of chemical and physical reactions, all of which may be regarded as forms of human behavior. Human beings are, however, not wholly or merely machines; they are living organisms and as such share with the plants and the lower animals certain forms of behavior which it has not thus far, at any rate, been possible to reduce to the exact and lucid formulas of either chemistry or physics.