The first principle in the Cultivation of Perception is the correct use of the Attention. The intelligent control of voluntary attention is a prerequisite to clear and distinct perception. We have called your attention to this matter in the preceding chapter. Halleck says: "A body may be imaged on the retina without insuring perception. There must be an effort to concentrate the attention upon the many things which the world presents to our senses.... Perception, to achieve satisfactory results, must summon the will to its aid to concentrate the attention. Only the smallest part of what falls upon our senses at any time is actually perceived." The sense of sight is perhaps the one of the greatest importance to us, and accordingly the cultivation of Perception with regard to impressions received through the eye is the most important for the ordinary individual. As Kay says: "To see clearly is a valuable aid even to thinking clearly. In all our mental operations we owe much to sight. To recollect, to think, to imagine, is to see internally,--to call up more or less visual images of things before the mind. In order to understand a thing it is generally necessary to see it, and what a man has not seen he cannot properly realize or image distinctly to his mind.... It is by the habitual direction of our attention to the effects produced upon our consciousness by the impressions made upon the eye and transmitted to the sensorium that our sight, like our other senses, is trained." Bain says: "Cohering trains and aggregates of the sensations of sight make more than any other thing, perhaps more than all other things put together, the material of thought, memory and imagination." Vinet says: "The child, and perhaps the man as well, only knows well what is shown him, and the image of things is the true medium between their abstract idea and his personal experience." This being the case, advice concerning the Cultivation of Perception must needs be directed mainly to the cultivation of the perception of sight-impressions. Brooks says: "We should acquire the habit of observing with attention. Many persons look at objects with a careless, inattentive eye. We should guard against the habit of careless looking. We should fix the mind upon the object before us; we should concentrate the attention upon that upon which we are looking. Attention, in respect to Perception, has been compared to a burning glass; hold the sun-glass between the sun and a board and the concentrated rays will burn a hole through the latter. So attention concentrates the rays of perceptive power and enables the mind to penetrate below the surface of things."