7. To make still clearer the fact that ultimate physical truths are, and must be, accepted as _a priori_, I pointed out that in every experiment the physicist tacitly assumes a relation between cause and effect, such that, if one unit of cause produces its unit of effect, two units of the cause will produce two units of the effect; and I argued that this general assumption included the special assumption asserted in the Second Law of Motion. . . . No reply: that is to say, no endeavour to show the untruth of this statement, but a quibble based on my omission of the word “proportionality” in places where it was implied, though not stated. 8. Attention was drawn to a passage {319} from Sir John Herschel’s _Discourse on the_ _Study of Natural Philosophy_, in which the “proportionality of the effect to its cause in all cases of _direct unimpeded_ action” is included by him among “the characters of that relation which we intend by cause and effect;” and in which this assumption of proportionality is set down as one _preceding_ physical exploration, and not as one to be established by it . . . No reply. 9. Lastly, a challenge to prove this proportionality. “It is required to establish the truth that there is proportionality between causes and effects, _by a process which nowhere assumes_ that if one unit of force produces a certain unit of effect, two units of such force will produce two units of such effect.” . . . No reply. Thus on all these essential points my three mathematical opponents allow judgment to go against them by default. The attention of readers has been drawn off from the main issues by the discussion of side issues. Fundamental questions have been evaded, and new questions of subordinate kinds raised. What is the implication? One who is able to reach and to carry the central position of his antagonist, does not spend his strength on small outposts. If he declines to assault the stronghold, it must be because he sees it to be impregnable. * * * * *