1907-8.] SPACE AND Its DIMENSIONS. B28 
of princes and heroes. An unkind fairy threw an undesirable present into 
its cradle, namely, the problem, or better the alleged problem, of the measur- 
abilitv of psychical magnitudes. Everybody was accustomed, for reasons 
which I have never been able to see, to exclude extension from the 
territory of the psychical and assign to the latter only the qualitative and 
the intensive. The opponents of experimental psychology took refuge 
with Kant to whom of course the modern problems of the quantitative 
relations between the physical and the psychical were as foreign as smoke- 
less powder was to the medieval knights, but who had said somewhere 
that psychology—by which he understood something totally different 
from that which we mean when we use the term—could never become an 
exact science. They forgot that the same Kant in a far more prominent 
passage of his critique laid down as one of the most important foundations 
of experience, indeed as an anticipation of every perception (rather loosely 
connected with his categories of quality) the axiom: Every phenomenon 
has an intensive magnitude, a degree. What is a degree other than an 
observable and measurable magnitude? 
The adversaries of the new science raised the question: can psychical 
magnitudes be measured? At the base of this way of putting the question 
lies the taking for granted of the fundamental distinction of the physical 
and the psychical. The physical according to the popular view comprises 
extensive as well as intensive magnitudes. The psychical had only 
intensive magnitudes. Now the question would be, either: 1.—Can 
intensive psychical magnitudes be measured? or 2.—Can intensive 
magnitudes be measured at all? If the former version is preferred, the 
opponents of experimental psychology must either establish a difference 
between physical and psychical intensities, or they must agree that their 
blow hits physics as well as psychology. With regard to this we might ask 
—What is intensity physically? Is for instance light-intensity physical 
or psychical? Has the physicist any right at all to speak of intensity? 
If he makes up his world of little particles, atoms, ions, or electrons which 
move in space, if he does not take refuge in dynamical suppositions which 
at once admit the miraculous or inexplicable, he will have to define his 
intensity in terms of extensive magnitudes as velocity, acceleration, 
amplitude, etc. On the other hand if the physicist or astronomer makes 
photometrical investigations, is his procedure and the criterion of his 
judgment essentially different from that of the psychologist when in- 
vestigating light intensities? Do they not both rely in the last instance 
on a comparison of psychical facts, states of consciousness? Does not 
every photometrical experiment rest on the subjective decision, which of 
two perceptions is the brighter? And does not always a least observable 
difference remain as an irreducible residuum? Is not the classification of 
