To such as these it will doubtless seem a marvelous thing for me to endeavor to treat by a geometrical method the vices and follies of men, and to desire by a sure method to demonstrate those things which these people cry out against as being opposed to reason, or as being vanities, absurdities, and monstrosities. The following is my reason for so doing. Nothing happens in Nature which can be attributed to any vice of Nature, for she is always the same and everywhere one. Her virtue is the same, and her power of acting; that is to say, her laws and rules, according to which all things are and are changed from form to form, are everywhere and always the same; so that there must also be one and the same method of understanding the nature of all things whatsoever, that is to say, by the universal laws and rules of Nature. The emotions, therefore, of hatred, anger, envy, considered in themselves, follow from the same necessity and virtue of Nature as other individual things; they have therefore certain causes through which they are to be understood, and certain properties which are just as worthy of being known as the properties of any other thing in the contemplation alone of which we delight. I shall, therefore, pursue the same method in considering the nature and strength of the emotions and the power of the mind over them which I pursued in our previous discussion of God and the mind, and I shall consider human actions and appetites just as if I were considering lines, planes or bodies. _Definitions_ I.--I call that an adequate cause whose effect can be clearly and distinctly perceived by means of the cause. I call that an inadequate or partial cause whose effect cannot be understood by means of the cause alone. II.--I say that we act when anything is done, either within us or without us, of which we are the adequate cause, that is to say (by the preceding Definition), when from our nature anything follows, either within us or without us, which by that nature alone can be clearly and distinctly understood. On the other hand, I say that we suffer when anything is done within us, or when anything follows from our nature, of which we are not the cause excepting partially. III.--By emotion I understand the modifications of the body, by which the power of acting of the body itself is increased, diminished, helped, or hindered, together with the ideas of these modifications. If, therefore, we can be the adequate cause of any of these modifications, I understand the emotion to be an action, otherwise it is a passion. _Postulates_ 1.--The human body can be affected in many ways by which its power of acting is increased or diminished, and also in other ways which make its power of acting neither greater nor less. 2.--The human body is capable of suffering many changes, and, nevertheless, can retain the impressions or traces of objects, and consequently the same images of things. _The Two States of Mind: Active and Passive_