Certain monads reach a state of development, or manifestation of activity, which is characterized by the possession of distinct organs. Such monads may be called, in a pre-eminent sense, "souls," and include all the higher animals as well as man. This possession of differentiated organs finds its analogue in the internal condition of the monad. What appears externally as an organ of sense appears ideally as a conscious representative state which we call "sensation." "When," Leibniz says, "the monad has its organs so developed that there is relief and differentiation in the impressions received, and consequently in the perceptions which represent them, we have feeling or sensation; that is, a perception accompanied by memory," to which at other times he adds "attention." Life, he says, "is a perceptive principle; the soul is sensitive life; mind is rational soul." And again he says in substance that when the soul begins to have interests, and to regard one representation as of more value than others, it introduces relief into its perceptions, and those which stand out are called "sensations." This origin of sensations as higher developments of the representative activities of a monad conditions their relation to further processes of knowledge. The sensations are confused knowledge; they are ideas in their primitive and most undifferentiated form. They constitute, as Leibniz somewhere says, the vertigo of the conscious life. In every sentient organism multitudes of sensations are constantly thronging in and overpowering its distinct consciousness. The soul is so flooded with ideas of everything in the world which has any relation to its body that it has distinct ideas of nothing. Higher knowledge, then, does not consist in compounding these sensations; that would literally make confusion worse confounded. It consists in introducing distinctness into the previously confused sensations,--in finding out what they mean; that is, in finding out their bearings, what they point to, and how they are related. Knowledge is not an external process performed upon the sensations, it is the development of their internal content.