It is very strange that, while psychologists have for so long a time studied the laws of association, no one has investigated whether the inverse process, dissociation, also has not laws of its own. We can not here attempt such a task, which would be outside of our province; it will suffice to indicate in passing two general conditions determining the association of series. First, there are the internal or subjective causes. The revived image of a face, a monument, a landscape, an occurrence, is, most often, only partial. It depends on various conditions that revive the essential part and drop the minor details, and this "essential" which survives dissociation depends on subjective causes, the principal ones of which are at first practical, utilitarian reasons. It is the tendency already mentioned to ignore what is of no value, to exclude that from consciousness. Helmholtz has shown that in the act of seeing, various details remain unnoticed because they are immaterial in the concerns of life; and there are many other like instances. Then, too, emotional reasons governing the attention orientate it exclusively in one direction--these will be studied in the course of this work. Lastly, there are logical or intellectual reasons, if we understand by this term the law of mental inertia or the law of least resistance by means of which the mind tends toward the simplification and lightening of its labor. Secondly, there are external or objective causes which are variations in experience. When two or more qualities or events are given as constantly associated in experience we do not dissociate them. The uniformity of nature's laws is the great opponent of dissociation. Many truths (for example, the existence of the antipodes) are established with difficulty, because it is necessary to break up closely knit associations. The oriental king whom Sully mentions, who had never seen ice, refused to credit the existence of solid water. A total impression, the elements of which had never been given us separately in experience, would be unanalyzable. If all cold objects were moist, and all moist objects cold; if all liquids were transparent and all non-liquids opaque, we should find it difficult to distinguish cold from moisture and liquidity from transparency. On his part, James adds further that what has been associated sometimes with one thing and sometimes with another tends to become dissociated from both. This might be called a law of association by concomitant variations.[7]