in objective terms_, and _mental facts are physical facts expressed in subjective terms_.” 7. But while Philosophy thus replaces the conceptions of Dualism and Idealism by the conception of the Two-fold Aspect, the special sciences in their analytical career have disregarded the problem altogether. The mechanical theory of the universe not only simplified research by confining itself solely to the objective aspect of phenomena, but by a further simplification set aside all vital and chemical relations, to deal exclusively with mechanical relations. In ascertaining the mathematical relations of the planetary system, no elucidation could possibly be gained from biological or chemical conceptions; the planets therefore were provisionally stripped of everything not mechanical. In systematizing the laws of motion, it was necessary to disengage the abstract relations from everything in any way resembling spontaneity, or extra-mechanical agency: Matter was therefore, by a bold fiction, declared to be inert, and its Motion regarded as something superadded from without. 7_a_. And this was indispensable for the construction of those ideal laws which are the objects of scientific research. Science, as we often say, is the systematization of Experience under the forms of ideal constructions. Experience implies Feeling, and certain fundamental Signatures, all reducible to the primary discernment of Likeness and Unlikeness. Hence Science is first a _classification_ of qualities or discerned likenesses and differences; next a _measurement_ of quantities of discerned likenesses and differences. Although measurement is itself a species of classification, it is distinguished by the adoption of a standard unit of comparison, which, being precise and unvarying, enables us to express the comparisons in precise and unvarying symbols. Whether the unit of length adopted be an inch, a foot, a yard, a mile, the distance of the earth from the sun, or the distances of the fixed stars, the quantities thus measured are symbols admitting of one invariable interpretation. The exactness of the mathematical sciences is just this precision and invariability of their symbols, and is not, as commonly supposed, the source of any superior certainty as to the facts. The classificatory sciences, which deal with qualities rather than with quantities, may be equally _certain_, and represent fuller _knowledge_, because involving more varied feelings, but they cannot pretend to exactness. Even on the quantitative side, certainty is not identical with exactness. I may be quite certain that one block of marble is larger than another--meaning that it affects me more voluminously--but I cannot know how much larger it is, without interpreting my feelings by the standard of quantity--the how-muchness as represented by that standard. The immense advantages of exact measurement need not be insisted on. The Biological Sciences, which