Page:Philosophical Review Volume 2.djvu/696

682 All measurements, physical as well as psychological, consist ultimately in the comparison of two phenomena of consciousness, generally two sensations. All physical measures have been developed out of psychological estimates. We measure temperature by noting the agreement of the length of the column of mercury with a certain portion of the scale; we measure the strength of an electric current by noting the angle through which the mirror is deflected or through which the needle passes, and this very angle is in turn measured by some length; we measure time by the agreement of the angle over which the hand has passed with a mark denoting the end of another angle taken as a standard.

Some confusion has been caused by the statement that psychologically we are able to judge only likenesses or difference, with the implication that in physics we are able to do something more, namely, to measure one object as a fraction or a multiple of another. Even if in physics we could directly do this, it would mean nothing more than that we could do so in psychology also, as when we compare two physical lengths we are only performing a psychological process. When we say that one line is apparently three times as long as another, we simply mean that the two mental pictures bear that relation, or that the series of muscular sensations produced by running the eye over the lines bear that relation. The fact is, however, that in the absence of graduated scales we express one quantity as a multiple of another only by estimates directly from our sensations. The graduated scales, by means of which we always work wherever possible, and by means of which we can obtain the accuracy of modern science, are really only records of direct judgments of likeness or difference. The 0 point on the thermometer means that the mercury column occupied that place under certain definite conditions, namely, immersion in the water of melting ice; the 100 mark means that the column was just so long when the thermometer was surrounded by steam at 760 mm. barometric pressure. We usually divide the intermediate space into 100 parts, but these