I think I'm only one among many who have a diary as their most precious possession, but, as a rule of thumb, I would think that people value objects in terms of how replaceable they are.

a 是不定冠词，表示"一个"或"一种"。 rule 的意思是"规则"或"法则"。 of 是介词，表示所属关系或连接两个名词。 thumb 的意思是"拇指"。 a rule of thumb 是一个固定短语，字面意思是"拇指规则"，实际指"经验法则"或"粗略估计的方法"，即根据经验而非精确计算得出的实用准则。