812 
thought. All thatit seems to be more than a 
detailed account of facts arises from the neces- 
sity of performing its function in an economical, 
even though symbolical, way (p. 154). 
The empirical, phenomenalistic standpoint 
above defined is, perhaps, as satisfactory a 
basis for scientific development as can at pres- 
ent be formulated. It would, of course, be hasty 
to regard it as a final formulation. That it 
contains inherent difficulties must have been as 
evident to the author as it is to the critic. For 
example, when it is said ‘the world consists 
only of our sensations’ one naturally wonders 
where the frequently mentioned ‘ connections’ 
between sensations are tocomein. Or, again 
(the historical stumbling block of ‘monism’), 
what is this experiential, or phenomenal, or 
sensational character, which, if everything 
possess it in common, would seem to be as diffi- 
cult to define as a scholastic ‘summum genus’ ? 
Finally that ‘self’ which is a group of sensa- 
tions, an object of experience, conveniently 
separated on the basis of certain peculiarities it 
may possess from other objects, is left in unde- 
fined relation to that subject or observer by 
whom and for whose ‘ economic’ ends the sepa- 
ration is made. Again the historic difficulties 
surrounding the relation of the self as subject 
to the self as object ! 
Nevertheless, it is not probable that any 
other theory removes these difficulties, although 
many seem more profoundly conscious of their 
existence. Their clearness and practicability 
have made views similar to those of Mach, 
favorite among scientists (cf. Munsterberg, 
Karl Pearson, et al.). Doubtless more elaborate 
care expended on the foundation would be out 
of proportion to the strength of the present 
scientific superstructure that rests upon it. 
From the consideration of these general ques- 
tion Mach turns to his special problem, the 
psychology of sensation. The ideal of his ef- 
fort he holds to be the determination of the 
‘connection of the psychologically observable 
data with the corresponding physical (physio- 
logical) processes’ (p. 29). The guiding thread 
of such a research must be ‘the principle of the 
complete parallelism of the psychical and 
physical’ (p. 30). 
It is but a step further to assume as many 
SCIENCE. 
(N.S. Von. VI. No. 152. 
physico-chemical neural processes as there are 
distinguishable qualities of sensation (Pref. to 
Eng. ed.). Similar principles of research are 
largely admitted and follow naturally from the 
concepts of Bell and Muller. But the question 
remains open as to whether different qualities of 
sensation are to be related to numerically dif- 
ferent structures, or whether likeness and differ- 
ence of mental status are to be related to quali- 
tative likenessand difference of physico-chem- 
ical processes taking place in nervous structures 
whose numerical distinctness, even if ex- 
istent, is accidental. Mach declares himself 
for the latter hypothesis, and an illustration is 
furnished in his treatment of tone-sensations. 
If, in this field, we look, with Helmholtz, for a 
special end organ corresponding to each pitch, 
likeness and difference of tones mean the ex- 
citation of the same and of different end organs, 
a@ point of much consequence in the theory of 
harmony. If, on the other hand, we regard 
the quality of the process that takes place in an 
end organ as a factor in the result, we might 
assume with Mach that every end organ is the 
seat of two kinds of energies (say a ‘Dull’ and a 
‘Clear’), the various ratios in which the two are 
excited being the physiological determinant of 
the pitch (pp. 128, 143, etc.). Two notes may 
then be more or less alike, although dependent 
upon numerically distinct end organs, since 
each involves the same two specific ‘ energies,’ 
only in different ratios. Mach is thus enabled 
to give a physiological basis to that theory of 
harmony which makes it depend upon the pres- 
ence of common overtones. All tones are more 
or less alike; harmonious tones are more alike 
than others, because their common (physiolog- 
ical) overtones reinforce each other (p. 144, ff.). 
The particular tone theory presented by Mach, 
although highly ingenious, and although it has 
won for itself some protagonists (e. g., Wundt), 
becomes a little strained when we try to ex- 
plain why two simple tones of different pitch 
are not identical with one tone of intermediate 
pitch (p. 129). Still the general principle of 
sense physiology, on which the theory rests, is 
of the greatest interest. Those who would object 
that progress in the physiology of the senses 
has been in the direction of differentiating nu- 
merically distinct structures corresponding to 
