Life has so many modes of being that we cannot know them all. We must choose the most primordial and it is easy to go astray in our choice. Thinking matter is far more complex than inert matter. Men's reciprocal relations are less easily defined than those of molecules or atoms. We must be careful not to lose our way among the host of tendencies, needs, appetites and desires we are apt to consider fundamental. Contingent aspects of individual or collective life often appear as necessary: man is easily tempted to think the forms of life which he desires are intended by nature. In the sixteenth century, Bodin was already teaching that the natural law forbids a sovereign to annex his subjects' possessions. The Physiocrats were also presuming that the human world was built according to laws analagous to those of physics. These laws appeared to them precisely the ones which guaranteed economic prosperity. They therefore taught that, in pursuing one's private interest one was, of necessity, working in the interest of all. Adam Smith raised the desire for gain to the dignity of a natural law. Ignorant of scientific method, the eighteenth century economists thought they were able to discover the secret of human relations as easily as the scientists discovered that of the relations of things. Jeremy Bentham imagined himself to be doing for the science of man what Newton had done for the science of matter. The Marxists, even more than the Utilitarians, claimed to use the scientific method in working out their doctrine. But neither Marx, Engels nor Lenin had any experience of scientific research. They ignored the very existence of operational concepts. Unsuspectingly, they mixed up two types of mental discipline; they confused a philosophic interpretation of life with the science of man. Thus Marxism, like Liberalism, put economics before everything else. Such errors show how careful we must be to distinguish which laws of life are really fundamental.
There are, of course, certain characteristics of the individual and the race which are indisputably real and universal. An immediate datum of observation is that everyone in good physical and mental health wants to remain alive. The number of suicides is relatively extremely small. It is equally certain that living creatures are irresistibly driven to reproduce themselves. Nor can one doubt that mind has progressively emerged from living matter in the course of the evolution of the animal species. Equally, in every individual, progressive development of consciousness takes place from the moment of birth to the threshold of old age. From these three orders of phenomena three inseparable, yet distinct, laws naturally follow; those of the conservation of life, the propagation of the race and the development of the mind. Though these laws are in fact, like philosophical principles, abstractions, they are abstractions very close to the concrete and still impregnated with reality. Though they cannot be expressed in mathematical formulas, they are legitimate children of the scientific method. They are obviously derived from the analysis of the amazingly complex activities of animals and men. We can trust them as surely as we trust the laws of gravity and of the conservation of energy.