[2] It is by no means unusual for modern scientists to deny the truth of this characterization, so far as regards this alleged recourse to the concept of causation. They deny that such a concept--of efficiency, activity, and the like--enters, or can legitimately enter, into their work, whether as an instrument of research or as a means or guide to theoretical formulation. They even deny the substantial continuity of the sequence of changes that excite their scientific attention. This attitude seems particularly to commend itself to those who by preference attend to the mathematical formulations of theory and who are chiefly occupied with proving up and working out details of the system of theory which have previously been left unsettled or uncovered. The concept of causation is recognized to be a metaphysical postulate, a matter of imputation, not of observation; whereas it is claimed that scientific inquiry neither does nor can legitimately, nor, indeed, currently, make use of a postulate more metaphysical than the concept of an idle concomitance of variation, such as is adequately expressed in terms of mathematical function. The contention seems sound, to the extent that the materials--essentially statistical materials--with which scientific inquiry is occupied are of this non-committal character, and that the mathematical formulations of theory include no further element than that of idle variation. Such is necessarily the case because causation is a fact of imputation, not of observation, and so cannot be included among the data; and because nothing further than non-committal variation can be expressed in mathematical terms. A bare notation of quantity can convey nothing further. If it were the intention to claim only that the conclusions of the scientists are, or should be, as a matter of conservative caution, overtly stated in terms of function alone, then the contention might well be allowed. Causal sequence, efficiency or continuity is, of course, a matter of metaphysical imputation. It is not a fact of observation, and cannot be asserted of the facts of observation except as a trait imputed to them. It is so imputed, by scientists and others, as a matter of logical necessity, as a basis of a systematic knowledge of the facts of observation.