But it is already clear from the above facts that our human sense-activity is limited, not only in quantity, but in quality also. We can thus only perceive with our senses, especially with the eye and the sense of touch, a part of the qualities of the objects in our environment. And even this partial perception is incomplete, in the sense that our organs are imperfect, and our sensory nerves, acting as interpreters, communicate to the brain only a translation of the impressions received. However, this acknowledged imperfection of our senses should not prevent us from recognizing their instruments, and especially the eye, to be organs of the highest type; together with the thought-organs in the brain, they are nature's most valuable gift to man. Very truly does Albrecht Rau say: "All science is sensitive knowledge in the ultimate analysis; it does not deny, but interpret, the data of the senses. The senses are our first and best friends. Long before the mind is developed the senses tell man what he must do and avoid. He who makes a general disavowal of the senses in order to meet their dangers acts as thoughtlessly and as foolishly as the man who plucks out his eyes because they once fell on shameful things, or the man who cuts off his hand lest at any time it should reach out to the goods of his neighbor." Hence Feuerbach is quite right in calling all philosophies, religions, and systems which oppose the principle of sense-action not only erroneous, but really pernicious. Without the senses there is no knowledge--"_Nihil est in intellectu, quod non fuerit in sensu_," as Locke said. Twenty years ago I pointed out, in my chapter "On the Origin and Development of the Sense-Organs,"[32] the great service of Darwinism in giving us a profounder knowledge and a juster appreciation of the senses. The thirst for knowledge of the educated mind is not contented with the defective acquaintance with the outer world which is obtained through our imperfect sense-organs. He endeavors to build up the sense-impressions which they have brought him into valuable knowledge. He transforms them into specific sense-perceptions in the sense-centres of the cortex of the brain, and combines them into presentations, by association, in the thought-centres. Finally, by a further concatenation of the groups of presentations he attains to connected knowledge. But this knowledge remains defective and unsatisfactory until the imagination supplements the inadequate power of combination of the intelligence, and, by the association of stored-up images, unites the isolated elements into a connected whole. Thus are produced new general presentative images, and these suffice to interpret the facts perceived and satisfy "reason's feeling of causality."