=Voluntary Attention Depends on Problem.=--It is evident from the foregoing that the characteristic of voluntary attention is not the absence or the presence of any special degree of interest, but rather the conception of some end, or purpose, to be reached in and through the attentive process. In other words, voluntary attention is a state of mind in which the mental movements are not drifting without a chart, but are seeking to reach a set haven. A person who is greatly interested in automobiles, for instance, on seeing a new machine, may allow his attention to run now to this part of the machine, now to that, as each attracts him in turn. Here no fixed purpose is being served by the attentive process, and attention may pass from part to part in a non-voluntary way, the person's general interest in automobiles being sufficient to keep the attention upon the subject. Suddenly, however, he may notice something apparently new in the mechanism of the machine, and a desire arises to understand its significance. This at once becomes an end to which the mind desires to attain, and voluntary attention proceeds to direct the mental movements toward its attainment. To suppose, however, that the interest, manifest in the former mental movements, is now absent, would evidently be fallacious. The difference lies in this, that at first the attention seemed fixed on the object through a general interest only, and drifted from point to point in a purposeless way, while in the second case an interesting end, or purpose, controlled the mental movements, and therefore made each movement significant in relation to the whole conscious process. =Attention and Knowledge.=--Mention has already been made of the relation of attention to interest. It should be noted, further, that the difference in our attention under different circumstances is largely dependent upon our knowledge. The stonecutter, as he passes the fine mansion, gives attention to the fretted cornice; the glazier, to the beautiful windows; the gardener, to the well-kept lawn and beds. Even the present content of the mind has its influence upon attention. The student on his way to school, if busy with his spelling lesson, is attracted to the words and letters on posters and signs. If he is reviewing his botany, he notes especially the weeds along the walk; if carrying to his art teacher, with a feeling of pride, the finished landscape drawing, his attention goes out to the shade and colour of field and sky. That such a connection must exist between knowledge and attention is apparent from what has been already noted concerning the working of the law of apperception.