The hidden fire in quicklime

Augustine (354–430), a Latin-speaking theologian from Roman North Africa and bishop of Hippo, sometimes chooses examples in The City of God that would have been familiar to a builder. One of them is quicklime. A dry piece gives little outward indication of the heat that can develop when water reaches it. For Augustine that discrepancy is precisely the reason to give the material more than a passing glance.

A material that can appear cold

And therefore we call it quicklime, the invisible fire being as the soul of that visible body. But the wonder is that when it is killed it is quickened.

– Augustine of Hippo, The City of God, 1909, p. 296.^[1]

The expression for living lime allows him a play on words. We imagine an invisible soul belonging to a visible body. In a similar way, he suggests, the stone conceals something that discloses itself only in its effects. When water is added those effects become difficult to miss. A substance commonly used to put out a flame here provokes the release of heat. The observer’s ordinary expectation points in the wrong direction at the very moment it seems most reliable.

How far the analogy with a soul goes

Augustine introduces the comparison during a discussion of what people may reasonably consider possible. Someone acquainted only with dry lime could reject an accurate description of its behaviour as incredible. This does not require Augustine to classify it as a living creature. The language of a soul gives a familiar shape to the idea of a hidden but effective property. The experience of heat demonstrates the distance between a confident first impression and a full account of what a body can do.