LECTURE III

§ I. Estimation of the Intensity of Sensation. § 11. Mathematical
Expression of the Law of Sens-^tion-intensities. § III. Signi¬
ficance OF Neg-ative Sens.ation-values ; Unit of Stimulus and
Unit of Sensation.

§1

THE question might, with some show of reason, be raised
as to whether the law which we have discovered is valid
for our quantitative estimation of sensation-magnitudes in general,
or whether it possesses only a more limited importance. For
all that we have directly ascertained is this : in what proportion
the just noticeable sensation-difference stands to the stimulus-
increment which conditions it. But, as a matter of fact, it will
be easily seen that the determination of this proportion is simply
a special case in the determination of a more general relation
of dependency.

No one will doubt that it is possible to pass gradually by
very small sensation-differences to very large ones. Suppose
that we take a sensation which has increased by a just notice¬
able magnitude, and that we allow this second sensation to
increase again by a just noticeable difference ; the difference
between the first and third will be clearer than that between
the first and the second. And if we proceed in this way, always
increasing by a just noticeable increment, we shall finally arrive
at a sensation-intensity which is very much greater indeed than
that of the sensation from which we set out. And we shall
have correspondingly reached a very considerable difference of
stimulus-intensity. Had we passed directly from the weak
stimulus to the strong, and therefore from the weak to the
strong sensation, we should never have been able to gain any
exact information as to the dependency of sensation upon
stimulus. Taking steps of such length from sensation to sensa¬
tion, we should   not  have  been  able to  decid,e  whether the

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