Programme of the chapter. Elementary feelings are innate. The question refers to their combinations. What is meant by 'experience'. Spencer on ancestral experience. Two ways in which new cerebral structure arises: the 'back-door' and the 'front-door' way. The genesis of the elementary mental categories. The genesis of the natural sciences. Scientific conceptions arise as accidental variations. The genesis of the pure sciences. Series of evenly increasing terms. The principle of mediate comparison. That of skipped intermediaries. Classification. Predication. Formal logic. Mathematical propositions. Arithmetic. Geometry. Our doctrine is the same as Locke's. Relations of ideas _v._ couplings of things The natural sciences are inward ideal schemes with which the order of nature proves congruent. Metaphysical principles are properly only postulates. Æsthetic and moral principles are quite incongruent with the order of nature. Summary of what precedes. The origin of instincts. Insufficiency of proof for the transmission to the next generation of acquired habits. Weismann's views. Conclusion. INDEX. PSYCHOLOGY. CHAPTER XVII. SENSATION. After inner perception, outer perception! The next three chapters will treat of the processes by which we cognize at all times the present world of space and the material things which it contains. And first, of the process called Sensation. SENSATION AND PERCEPTION DISTINGUISHED. _The words Sensation and Perception_ do not carry very definitely discriminated meanings in popular speech, and in Psychology also their meanings run into each other. Both of them name processes in which we cognize an objective world; both (under normal conditions) need the stimulation of incoming nerves ere they can occur; Perception always involves Sensation as a portion of itself; and Sensation in turn never takes place in adult life without Perception also being there. They are therefore names for different cognitive _functions_, not for different sorts of mental _fact_. The nearer the object cognized comes to being a simple quality like 'hot,' 'cold,' 'red,' 'noise,' 'pain,' apprehended irrelatively to other things, the more the state of mind approaches pure sensation. The fuller of relations the object is, on the contrary; the more it is something classed, located, measured, compared, assigned to a function, etc., etc.; the more unreservedly do we call the state of mind a perception, and the relatively smaller is the part in it which sensation plays.