brain, and that the sensations excited through these organs constitute the raw material of the mental life—touch, hearing and sight being recognized as par excellence intellectual senses. Now, it is a most significant fact, from the point of view of my third thesis, that the activities of these three senses in particular involve muscular co-operation as an essential accessory; and the profound relations which exist between many of the mental processes and muscular action are at least adumbrated in certain experimental observations by Wundt upon the eye. He has shown, for example, that vertical distances appear greater than equal horizontal distances in the proportion of 4·8 to 4, and that the same proportion exists between the muscular forces which move the eye vertically and those which move it horizontally; that the minimum of movement of the eye capable of exciting consciousness of contraction and the smallest perceptible distance are in exact agreement, both answering to an angle of one sixtieth of a degree; that we are able to distinguish a difference in length of two lines if it amount to one fiftieth of the entire length of the shorter one—the difference in movement of the eyes in this case being also one fiftieth of their entire linear movement.
These relations can not be mere coincidences. Ideas of the size and distance of objects are also attributable in part to the degree of muscular action involved; for the nearer an object to the eye, the greater the muscular exertion required in converging the axes of the balls upon the object, and the greater the tax upon the muscles of accommodation; and it is not the visual sensation alone which gives the idea of distance, although the degree of distinctness, no doubt, has a marked influence, but the muscular sensations excited by the movements of accommodation and convergence must also contribute to the result. A mere allusion to the immense importance of visual perceptions in our mental furnishings will sufficiently indicate the bearings of these facts on the relations of muscular activity to mental activity and growth. To the significance of the muscles as organs of the muscular sense must then be added that which is due to the existence of a muscular element in other sense-organs.
Since movements, no less than sensations, play a conspicuous part in the acquisition of knowledge of the external world, it follows that ideas are a revival of ideal movements as well as of ideal sensations. My fourth thesis is, therefore, that ideas have no special separate centers in the brain, but result from the excitation of those areas which have taken part in the original acquisition of ideas, viz., the sensory and motor centers. These two kinds of centers, with the inhibitory centers and numerous connecting
- See "German Psychology of Today," Ribot.