Diatribe de prodigiosis crucibus [Diatribe on prodigious crosses] (1661)

Diatribe de prodigiosis crucibus was written in response to widespread reports of cross-shaped signs appearing in the sky, on buildings, in natural materials, and within optical and meteorological phenomena in 1660. At a time when such signs were often interpreted as divine portents—harbingers of plague, war, or judgment—Kircher sought to subject them to systematic analysis rather than uncritical wonder. Drawing on geometry, optics, astronomy, and natural philosophy, the Diatribe assembles a vast comparative archive of cruciform appearances, asking how the form of the cross might arise from physical causes (e.g. volcanic ash and stains) while still retaining symbolic and theological significance. The work exemplifies a Jesuit mode of inquiry that aims to reconcile natural explanation with sacred meaning. Kircher does not deny the religious power of the cross; rather, he reframes prodigious signs as phenomena that operate at the intersection of nature, human perception, and divine providence. By classifying crosses according to material, optical, and geometric principles, he argues that God's signs do not violate natural order but are manifested through it. The Diatribe thus stands as both a treatise on prodigies and a defense of learned interpretation, insisting that wonder must be guided by disciplined knowledge rather than fear or superstition.