Christian missionaries branded Muslims irrational when rational arguments failed to convert them.

Evidence

The bitter experience of thirteenth-century missionaries was that the Saracens were unimpressed by their “rational” arguments; increasingly, they will conclude (as Peter the Venerable did for the Jews) that these Saracens must be irrational. This will indeed become the predominant stereotype in the fourteenth and fifteenth centuries. — John V. Tolan, Saracens, 1258

Categorization

• XREF: Genealogically grounds the Islam-as-irrational trope that Anidjar/Aziz trace in modern discourse — shows its medieval missionary origin in conversion failure rather than observation.
• ADDENDUM: Supports the complaints/immodesty essay’s method of tracing moral panics to identity signaling — here irrationality is likewise projected onto Muslims as retaliation for rejecting Christian truth.