Bergson, Time and Free Will, Chapter 1, §38 "The Psychophysicist Claims to Compare and Measure Sensations. Delboeuf's Experiments"

by Corry Shores
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[The following is summary; my commentary is in brackets.]

Bergson, Time and Free Will

(Essai sur les données immédiates de la conscience)

Chapter I, "The Intensity of Psychic States"

Part XIII: "Sensation of Light"

§38 "The Psychophysicist Claims to Compare and Measure Sensations. Delboeuf's Experiments"

Unlike the physicist, the Psychophysicist studies and measures sensations. Fechner, for example, integrates infinitely small differences in sensations. (55-56)

Delboeuf places an observer in front of three concentric rings which vary in brightness. By an ingenious arrangement he can cause each of these rings to pass through all the shades intermediate between white and black.

Then, we gradually and continuously lighten the tone of ring C, while keeping A and B the same. We continue to perceive the contrast between A and B as being the same, while noticing that the contrast between B and C is changing.

At this point we might say that B is just as different from A as it is from C. This produces a standard. So by using such an analogous comparison of differences, we might then construct a scale of luminous intensities.

But Bergson rejects the claim that the sensations of difference between the two comparisons are really equal. He argues that if they were truly equal, then they would be identical. But we know that in one case there is a perception of difference between the darkest and the intermediary tone, and in the other case there is a perception of difference between the intermediary tone and the lightest one. For Bergson, this is enough to say that they cannot be identical sensations.

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Images from the pages summarized above, in the English Translation [click on the image for an enlargement]:

Images from the pages summarized above, in the original French [click on the image for an enlargement]:

Bergson, Henri. Time and Free Will: An Essay on the Immediate Data of Consciousness, Transl. F. L. Pogson, (New York: Dover Publications, Inc., 2001).

Available online at:

http://www.archive.org/details/timeandfreewill00pogsgoog

French text from:

Bergson, Henri. Essai sur les données immédiates de la conscience. Originally published Paris: Les Presses universitaires de France, 1888.

http://www.archive.org/details/essaisurlesdonn00berguoft