The mind of man is so constituted that no change can take place in any material object without awakening the idea of some _cause_. An _effect_ is defined as "some change of state or mode of existence in matter or mind." A _cause_ is defined as "that without which no change would take place in matter or mind, and with which it will take place." As the ideas of cause and effect are so constantly conjoined in all our acts of perception, these ideas will return together in our conceptions. Thus, if we see an instrument which has been the cause of pain, the idea of this effect will be recalled by a conception of the cause; or if the mind is dwelling on the memory of some beautiful painting or poetry, the author of these works will probably recur to the mind in connection with these conceptions. We sometimes meet with persons of such peculiar habits and dispositions, that, whenever they are encountered, the feelings are wounded or the temper crossed by their ill-timed or ill-natured remarks. The conceptions of such persons will ordinarily be attended by the memory of some pains of which they have been the cause, and the mind will involuntarily shrink from contact with them, as from the points and thorns of a bramble-bush. Those events, therefore, or those objects which have the relation of _cause_ and _effect_ existing between them, will ordinarily be united as objects of conception. The mind of man is continually noticing the _relations_ which exist between the different objects of its conceptions. As no idea of relation can be gained without comparing two or more things together, those objects which are most frequently _compared_ will naturally be most frequently associated together in our conceptions. It has been shown that language is founded on that principle of the mind which enables us to notice certain qualities in things abstracted from other qualities, and to apply names to objects according as we find certain qualities united in them. Of course, in the use of language, the mind is continually led to notice the particulars in which objects resemble each other, and also the particulars in which they differ; consequently the mind, in learning and in applying names, is continually comparing objects, both to discover the particulars in which they are alike and those in which they differ, so that two objects are thus brought together before the mind.