So because you have classified something from one aspect do not imagine that you are necessarily precluded from classifying it from any other. A man who is studying the theory of money may divide the medium of exchange into standard money and credit currency. But this need not keep him from viewing it as coins, government notes, and bank currency, nor should it prevent him from classifying it into, say (1) hand-to-hand money, (2) written or printed orders of one party to pay specified sums to another, and (3) book accounts.[4] All these classifications will be true; all may be useful for a full comprehension. Every classification should of course be logical; but it is far more essential that it be utilizable. And while we are treating of utility, we might note that this _pragmatic_ method can be applied with profit to nearly all our positive problems. Before starting to solve a question—while deciding, for instance, on the validity of some nice distinction in logic—we should ask ourselves, “What practical difference will it make if I hold one opinion or the other? How will my belief influence my action?”—(using the word “action” in its broadest sense). This may often lead our line of inquiry into more fruitful channels, keep us from making fine but needless distinctions, help us to word our question more relevantly, and lead us to make distinctions where we really need them. We are now ready to consider in order a number of constructive methods in thinking. One method applicable to almost all problems is what we may call either the _deductive_ or the _à priori_ method. This method reaches a conclusion without observation or experiment. It consists in reasoning from previous experience or from established principles to particular facts. It may, however, be used to confirm observation and experiment as well as to take their place. Take the all important questions in biology of whether or not specific characteristics acquired by an animal during its life time are inherited by offspring. The a priori method would examine the structures of the body, the germ plasm from which the offspring develops, and the relation between them, and would ask just how a specific change in the body could affect the germ. If it were found that the tissues that are to continue the race were set off so completely from the structures of the body as to make inconceivable any manner by which they could be influenced by changes in these structures, then this method would decide that acquired characteristics are not transmitted.