In order to make clear in what way certain terms have been used in the preceding chapters, it may not be out of place to indicate how it is intended that they should be employed. The word “cause” has been used in the sense in which the physicist uses the term. A “stimulus” is the chain of effects of a cause acting on a living body. In certain cases the cause itself may be spoken of as the stimulus, but only when its specific action on a living body is implied. A “factor” is a more general term and is usually one or more of a number of causes that produce a result. It may prove convenient to use this term where a change in form is produced. Thus the size of a piece is one of the factors that determines the result; the part of the body from which the piece is taken may also be a factor, or rather the kind of material contained in the piece. These examples will suffice to show that the word is used for an observed connection of a very general sort, especially for those cases in which we have not analyzed the factor into its components. The term is especially useful for cases in which the change in form is the outcome of the innate properties of the organization. The term may be used so that it need not prejudice the result, either in favor of a physico-causal or a vitalistic-causal point of view. It may be convenient to use it as an indifferent term in these respects. The word “force” I have attempted to avoid as far as possible, except in such current expressions as “the force of gravity,” etc., for, apart from the loose way in which the word is used even by physicists, we know so little about the forces in the organism that it is best, I think, to use the word as sparingly as possible, and only where a known physical force can be shown to produce an effect.