2. _Material things and natural forces differ in kind and nature._ This is an axiom which we must take as a basis for reasoning in economics. Things have certain physical qualities quite apart from any action or influence of man. They are operated on by mechanical laws; the force of gravitation causes them to fall at a certain rate under given conditions. They differ in specific gravity, reflect the rays of light, absorb or transmit heat. All these things are for man ultimate physical facts, but unless he knows these facts he cannot take full advantage of the favorable qualities of things or weigh properly their importance to his welfare. Things differ in a multitude of ways in their chemical qualities. Niter, charcoal, and saltpeter, combined in certain proportions, give certain reactions; different combinations give various results. Solids combine to form gases, and liquids unite to form solids, and these qualities and reactions of material things are for man ultimate truths of chemistry. Likewise many things have certain physiological effects. Sunshine acts on living bodies, whether plant, animal, or man, in certain ways. Some plants are nourishing to man, others are poisonous. If man were not on the earth, things would have the same physical and chemical qualities, mechanical laws would be the same as at present so far as we can conclude. Man cannot change the nature of things; but he can acquaint himself with that nature and then put the things into the relations where a given result will follow. [Sidenote: But economics has to do with psychological effects] 3. _As a result of these differences, things have different relations to wants._ These various qualities, physical, chemical, physiological, are important in an economic sense only as they produce psychological effects, that is, as they affect the feelings and judgments of men. We come to some general thoughts which it will be well to define. [Sidenote: Some definitions] Gratification is the feeling that results when a want has been met. Feelings are hard to define in words; the best definition is found in the experience of each individual. We can only say, therefore, that gratification is the attainment of desire, the fulfilment of wants. The word that has usually been employed in this sense in economic discussion is "satisfaction"; but by its derivation and general usage satisfaction means "the complete or full gratification" of a desire, and this meaning is quite inconsistent with the thought in many connections in which the word is used. We shall therefore prefer here the word gratification, and its corresponding verb, gratify. Wealth is the collective term for those things which are felt to be related to the gratification of wants. The word is applied in economic discussion to any part of those things, no matter how small. We shall have occasion later to define and discuss this term more fully.