An 18th-century French jurist's maxim, preserved in Garraud's appendix to Esmein's 1913 volume, states directly that two half-proofs cannot make a full proof

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Appendix B ("History of the Continental System of Evidence") of the 1913 Esmein volume asks whether "indications" — partial or circumstantial signs of guilt, short of full testimonial or documentary proof — could be summed to reach the threshold required for conviction in ancient French criminal law. The appendix is not Esmein's own text. An editorial footnote at its head states: "[Appendix B = §XLVI of Professor Garraud's 'French Criminal Procedure.' For this author and work, see the Editorial Preface. — Ed.]" — so the reporting voice in this passage is R. Garraud's, carried in Esmein's volume by the editor, William E. Mikell. At §9 ("Proximate Indications"), printed p. 625, a footnote reports: "Certain jurists maintained that the indications might be added together. But this combination was usually rejected." It then quotes the 18th-century jurist Poullain du Parc directly, citing his op. cit., t. II, p. 116: "The half-proof… is no more conclusive than a half-truth, and for the same reason that two uncertainties cannot make a certainty, two half-proofs cannot make a full proof."

Esmein's own main text carries a parallel statement of the same maxim at pp. 264–266, inside his chapter on the procedure under the Ordinance of 1670 — but in different words and with no inline attribution to Poullain du Parc: there it reads "two half-proofs cannot constitute a complete proof." The wording this note's title turns on ("make a full proof") exists only at p. 625, in Garraud's appendix. Anything citing the maxim in that form must cite p. 625, not p. 265.

This is a named-jurist, Tier-1-sourced statement of the exact "two half-proofs do not sum to a full proof" doctrine that claim-two-half-proofs-combined-to-full-proof-except-in-grave-criminal-cases attributes, on weaker Tier-3/4 authority, to Roman-canon ecclesiastical procedure. It arrives via a wholly different tradition — the secular criminal practice of the French ancien régime, arguing about circumstantial "indications" rather than the witness-or-document semiplena probatio at the center of claim-roman-canon-law-rated-one-witness-equal-to-a-private-document — five centuries and a different court system removed from the encyclopedia's ecclesiastical framing. Poullain du Parc is independently identifiable as Auguste-Marie Poullain du Parc (1703–1782), advocate and later dean at the Parlement of Brittany and professor of French law at Rennes; the volume cites his Principes de droit français suivant les maximes du Parlement de Bretagne (Rennes, 1771), though it gives no dates for the man himself. It therefore corroborates that the non-additive principle was real and had named defenders and named opponents among practicing jurists, without confirming the encyclopedia's specific criminal/matrimonial/grave-civil three-way exception structure. That narrower question remains open at question-verify-half-proof-combination-rule-criminal-exception-primary.