Why the second heart does not follow
Life is not an illogicality; yet it is a trap for logicians.
The trap is a change in the status of a claim. The visitor observes several paired organs, promotes that pattern into a universal rule, and then treats a further pair as deductively guaranteed. His final deduction would follow if the universal premise were true; what the observations fail to establish is that premise. Chesterton's opening distinction therefore matters: life need not contradict logic to defeat someone who mistakes a persuasive pattern for a necessity.
Hume supplies a useful comparison, rather than an identified source. In his Enquiry concerning Human Understanding (first published in 1748 as Philosophical Essays concerning Human Understanding; inspected here in Hume Texts Online's transcription of the 1777 text), he distinguishes mathematical relations from matters of fact, whose opposites can be conceived without contradiction. He also asks why experience of particular objects should extend to other objects that may only appear similar, and insists that geometry cannot replace experience in establishing natural laws. These are pertinent distinctions even though his inquiry principally concerns causal inference, whereas Chesterton's example generalizes anatomical symmetry. See §4.1–2, 4.13, 4.16 and the edition notes.
This comparison qualifies the apparent opposition between Christianity and scepticism that follows: a sceptical philosopher can already warn against the visitor's confidence. The example establishes the need to discover where a pattern stops; by itself it does not establish that Christianity uniquely supplies that discovery. Its further role is architectural. The chapter later presents Christian ethics as an irregular arrangement of apparently conflicting commitments, and its closing image of precarious equilibrium answers this opening demand for accuracy without simple symmetry. That connection is an interpretation of the chapter's construction, not evidence that Chesterton borrowed Hume's argument. Orthodoxy, chapter VI, opening and closing paragraphs; inspected John Lane 1909 printing, opening pp. 146–47.
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