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Hermann von Helmholtz

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Now it is a universal law of the perceptions obtained through the senses that we pay only so much attention to the sensations actually experienced as is sufficient for us to recognise external objects. In this respect we are very one-sided and inconsiderate partisans of practical utility; far more so indeed than we suspect. All sensations which have no direct reference to external objects, we are accustomed, as a matter of course, entirely to ignore, and we do not become aware of them till we make a scientific investigation of the action of the senses, or have our attention directed by illness to the phenomena of our own bodies. Thus we often find patients, when suffering under a slight inflammation of the eyes, become for the first time aware of those beads and fibres known as mouches volantes swimming about within the vitreous humour of the eye, and then they often hypochondriacally imagine all sorts of coming evils, because they fancy that these appearances are new, whereas they have generally existed all their lives.
"On the Physiological Causes of Harmony" (1857), p. 81

Hermann von Helmholtz

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As you are aware, no perceptions obtained by the senses are merely sensations impressed on our nervous systems. A peculiar intellectual activity is required to pass from a nervous sensation to the conception of an external object, which the sensation has aroused. The sensations of our nerves of sense are mere symbols indicating certain external objects, and it is usually only after considerable practice that we acquire the power of drawing correct conclusions from our sensations respecting the corresponding objects.

Hermann von Helmholtz

According to the technical language of old writers, a thing and its qualities are described as subject and attributes; and thus a manís faculties and acts are attributes of which he is the subject. The mind is the subject in which ideas inhere. Moreover, the manís faculties and acts are employed upon external objects; and from objects all his sensations arise. Hence the part of a manís knowledge which belongs to his own mind, is subjective: that which flows in upon him from the world external to him, is objective.

William Whewell

The Common Sense, is that which judges of things offered to it by the other senses. The ancient speculators have concluded that that part of man which constitutes his judgment is caused by a central organ to which the other five senses refer everything by means of impressibility; and to this centre they have given the name Common Sense. And they say that this Sense is situated in the centre of the head between Sensation and Memory. And this name of Common Sense is given to it solely because it is the common judge of all the other five senses i.e. Seeing, Hearing, Touch, Taste and Smell. This Common Sense is acted upon by means of Sensation which is placed as a medium between it and the senses. Sensation is acted upon by means of the images of things presented to it by the external instruments, that is to say the senses which are the medium between external things and Sensation. In the same way the senses are acted upon by objects. Surrounding things transmit their images to the senses and the senses transfer them to the Sensation. Sensation sends them to the Common Sense, and by it they are stamped upon the memory and are there more or less retained according to the importance or force of the impression.

Leonardo da Vinci

Over his own heart and his own thoughts he watched attentively. He knew not whither his longing was carrying him. As he grew up, he wandered far and wide; viewed other lands, other seas, new atmospheres, new rocks, unknown plants, animals, men; descended into caverns, saw how in courses and varying strata the edifice of the Earth was completed, and fashioned clay into strange figures of rocks. By and by, he came to find everywhere objects already known, but wonderfully mingled, united; and thus often extraordinary things came to shape in him. He soon became aware of combinations in all, of conjunctures, concurrences. Erelong, he no more saw anything alone. ó In great variegated images, the perceptions of his senses crowded round him; he heard, saw, touched and thought at once. He rejoiced to bring strangers together. Now the stars were men, now men were stars, the stones animals, the clouds plants; he sported with powers and appearances; he knew where and how this and that was to be found, to be brought into action; and so himself struck over the strings, for tones and touches of his own.


All existing things upon this earth, which have knowledge of their own existence, possess, some in one degree and some in another, the power of thought, accompanied by perception, which is the awakening of thought by the effects of external objects upon the senses.

Augustus De Morgan
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