There are some words which recall only _one_ quality or circumstance, and which, therefore, can not be defined like the words which recall various qualities and circumstances, as joy, sorrow, color, and the like. Such words as these are defined by mentioning the times or circumstances when the mind is conscious of the existence of the idea to be recalled by the word. Thus _joy_ is "a state of mind which exists when any ardent desire is gratified." _Color_ is "a quality of objects which is perceived when light enters the eye." Those conceptions which can be defined by enumerating the several qualities and circumstances which compose them are called _complex ideas_, and the words used to designate them are called _complex terms_. Such words as landscape, wrestler, giant, and philosopher, are complex terms. The word landscape recalls a complex idea of various material things. The word wrestler recalls an idea of a material object and one of its actions. The word giant recalls an idea of a thing and its relation as to size. The word philosopher recalls the idea of a thing and one of its qualities. Those conceptions which are not composed of several qualities and circumstances, but are themselves a single quality or circumstance, are called _simple ideas_, and the words used to recall them are called _simple terms_. Such words as sweetness, loudness, depth, pain, and joy, are simple terms. Some terms which express emotions of the mind are entirely simple, such as sorrow, joy, and happiness. Others are words which recall an idea of a simple emotion and of its _cause_, such, for example, as _gratitude_, which expresses the idea of an emotion of mind and also that it was caused by some benefit conferred. Words that express simple ideas can be defined only by some description of the circumstances in which these ideas exist, or by a reference to their causes or effects. CHAPTER XII. ASSOCIATION. The causes of the particular succession of our ideas, and the control which the mind has in regulating this succession, is a subject no less interesting than important; for if by any act of choice the mind has the power of regulating its own thoughts and feelings, then man is a free agent and an accountable being; but if the conceptions and the emotions depend entirely upon the constitution of things, and thus, either directly or indirectly, on the will of the Creator, then man can not be accountable for that over which he can have no control. In the preceding chapter has been illustrated the effect which the co-existence of desire has in regard both to our sensations and our conceptions, tending to make those which are fitted to accomplish the object desired very vivid and prominent, while others, to a greater or less extent, disappear.