道理(dào lǐ) - sense, reason, principle in Mandarin

道理 (dào lǐ) is the sense in an argument, or a general principle that holds true. The workhorse pattern is 有道理: 你说得很有道理 is that is a fair point, and it is one of the most useful things you can say in a Chinese conversation. The negative is 没有道理 or the stronger 毫无道理.

The other common pattern is 讲道理, to reason with someone or to be reasonable. 他是个讲道理的人 means he is a reasonable man; 跟他讲不通道理 means you cannot reason with him. Note that 讲道理 is also what a parent does to a child instead of simply shouting.

Keep it apart from two neighbours. 理由 (lǐ yóu) is a reason in the sense of a justification for a specific act: 你迟到的理由是什么. 原因 (yuán yīn) is a cause. 道理 is neither of those; it is the underlying logic or rightness of a thing, closer to English "sense" than to "reason".