His proposal demonstrated great reasonableness, which won everyone's support.

reasonableness 是由形容词 reasonable（合理的）加上名词后缀 -ness 构成的名词形式，表示"合理性"或"合理程度"。reasonable 本身由名词 reason（理由、理性）加上形容词后缀 -able（能够...的）构成，字面意思是"能够讲道理的"。