8. Having thus proved that the argument of the Theist generally, as well as the particular one advanced by S. D. C. at p. 27, is, by being based on the erroneous statement that there is only one means known to human experience, of producing phenomena identical with those that are the product of design, and that this one is design itself; there being, on the contrary, two such means, one of which is not design; having, I say, proved that your argument, by being so based, is invalid, I find I must fully agree with you, that there is evidence of "an unmistakable cosmical unity." 9. The true inquiry, therefore, is, which of those two principles of order is, in the agency inquired into, the agent under these circumstances, and whether both, and how far, under our ignorance of what may be (a most important point that is carefully to be considered) we are entitled to affirm as indubitable, to denounce as contradictory, to advance as probable, to conjecture, to surmise, or to speculate on this question. THE TRUTH OF FIRST PRINCIPLES. 1. You ask "my idea on the impossibility of proving the truth of First Principles?" By "truth" you mean the ascertained existence of any idea or thing, and the ascertained consistency of any statement with some such idea or thing. By "principles" you mean not simply cardinal propositions, but cardinal propositions that we have ascertained to be true. By "first principles" you mean the indubitably true but unprovable elementary principles of all our knowledge. You mean that these principles are the ground whereon we build in our reasonings; all that we build upon them must, in consequence of being so built, admit of being "proved" whether we have built rightly--that is, admit of being subjected to the test whether the reasoning is correct; but these "first principles" are confessedly exempted from this test, and yet are received as true, no less than the others that have sustained this ordeal. You ask the meaning of this privilege, whether it is right; and, if so, to what propriety or necessity of the case it is due? 2. You ask, "How is truth ascertained to be truth?" or, in other words, "What is the criterion of truth?" With respect to the first query--In accordance with the definition I have above given of truth, it would seem that it must have two sources, experience and reason, experience who notifies the existence of certain ideas or things, and reason, who forms propositions suggested by them. Experience, therefore, acts the simple part of supplying all the materials of truth; while reason, besides his acknowledged office of judge of all truth, exercises the quite different function of being himself the purveyor of a portion of it. So indubitable is it that truth can have these two sources only, that even fanaticism would be found confessing the principle; while it appeals to the experience of those who agree with it, as well as professes to be reasonable.