To suppose that its cognitive value can be eked out or supplied by associating it with other sensory qualities of the same nature as itself, is like supposing that by putting apile of sand in the eye we can get rid of the irritation caused by asingle grain. To suppose, on the other hand, that we must appeal to asynthetic activity of an independent thought to give the quality meaning in and for knowledge, is like suppos- ing that by thinking in our heads we can convert apile of bricks into abuilding. Thinking, carried on inside the head, can make some headway in forming the f)lan of abuilding. But it takes actual operations to which the plan, as the fruit of thought, gives instrumental guidance to make abuilding out of separate bricks, or to transform an isolated sensory quality into asignificant clew to knowledge of nature.