As to the charge against the Materialists--whom Mr Pearson in another passage typifies by Büchner and Bradlaugh--it is practically untrue on one head, that of force being the "cause" of motion; and quite inconclusive on another, that of "enforcement" and "necessity." Mr Pearson is uncandid enough to cite no passage on either head, and I know not whether the latter is not as inaccurate as the other. Even if, however, a Materialist should talk of motion as a "necessity" of matter, it would amount to nothing to impugn him without showing what he conceives "necessity" to be. The word is a plexus of connotations; and to identify it out-of-hand with the conceptions of spiritists is a course more worthy of a theologian than of a man of science. Mr Pearson's way of talking of "enforcement," as if the word conveyed any fixed scientific sense whatever, is a commission of the very offence he unjustly charges on the school of Büchner. But as to the statement that Büchner and Bradlaugh are wont to speak of force as the "cause" of motion, it is really not true. Büchner in his typical work, "Force and Matter," does in one passage write somewhat unguardedly of the "force inherent in matter"--_i.e._ in the "something" empirically known "which we call matter"--as being the cause (_Ursache_) of the activities which are the phenomena of the said matter;[86] but this momentary verbal laxity is not at all the burden of his treatise. It is in any case much more pardonable than the gross contradictions which Mr Pearson quotes from the writings of Professors Thomson and Tait, collaborators in special physics; it is paralleled by phrases which he cites from Huxley, Nägeli, Spencer, and Weismann; and it is much less serious than the inconsistencies and fallacies into which Mr Pearson himself repeatedly falls. Even while repudiating the notion above cited as to "cause" (which he does without reference to the well-known discussions, from Hume onward, as to the force of the term), he writes (p. 352): "... We still shall not find in 'force,' _as_ either the cause of motion, or the cause of change in motion, _anything more_ than that routine of perceptions which ... is the scientific definition of causation." With this account of causation Büchner and Bradlaugh, and everybody else who has appreciated the effect of Hume's reasoning, would agree, save in so far as the phrasing falls into the very crudities of expression which mar Hume's pioneer argument. Mr Pearson writes that we "sadly need separate terms for the routine of sense-impressions," yet he never hesitates either to use a general term loosely or to disparage an unpopular man for doing the same thing. He says of material particles (p. 327): "All we can scientifically say is, that the _cause_ of their motion is their relative position; but this is no explanation of why they move in that position." This use of "cause" is really looser than Büchner's, and is not "scientific" at all.