Page:Mind (New Series) Volume 6.djvu/350

 334 B. EUSSELL : number, the conception may be any we please, and the con- tents to which it is applied are regarded as only numerically, not conceptually, different. They are collected into a whole of precisely similar contents, and comparison plays only the preliminary part of showing the same conception to be applicable to all the contents. In quantity, the common conception must, as we have just seen, be some quality, adjective, or relation things as such are not susceptible of quantitative treatment. Moreover, while number depends upon the conceptual identity of the contents numbered, quantity depends essentially upon their differences. A col- lection of quantities of the same kind, while they must be identical in quality, must also be susceptible of differences of quantity, and it is precisely these differences which are relevant. But with intensive quantities, which we are at present considering, these differences of quantity are not themselves quantities. The difference between two inten- sive quantities, in fact, differs from each as much as the difference between two horses differs from a horse. With this, it will be seen, we have lost the last possibility of numerical treatment : extensive quantities could be divided into parts, which could be counted, but intensive quantities cannot be numerically measured in any way. What, then, can we say about such quantities, and how are they to be treated ? l Since intensive quantities are not divisible or numerically measurable, the quantitative changes in qualities which have intensive quantity cannot be submitted to any objec- tive test, but must be judged by immediate comparison of sensations. Where changes of sensation are found em- pirically to be correlated with changes of some extensive quantity, we may agree conventionally, as we do with the thermometer, to take the magnitude of this correlated ex- tensive quantity as measuring the magnitude of our inten- sive quantity. But such an agreement can never be more than a convention, and the apparent objectivity which results is fictitious and merely convenient. All that we really have to go upon is the immediate apprehension of a change, as revealed by subjective comparison. This ex- plains why intensive quantity, if it is a category at all, is mainly a psychological category. To fix our ideas, let us, since intensive quantity appears to be really a matter for Psychology, take pleasure as the 1 On the subject of intensive quantity, compare Sigwart's Logic, vol. ii., 70.