There is a difficult class of terms, usually regarded as magnitudes, apparently implying relations, though certainly not always relational. These are differential coefficients, such as velocity and acceleration. They must be borne in mind in all attempts to generalize about magnitude, but owing to their complexity they require a special discussion. This will be given in Part V; and we shall then find that differential coefficients are never magnitudes, but only real numbers, or segments in some series.(§ 161 ¶ 1)