Picture pointing at one separate, self-contained item — a single thing you can pick up and count on its own, set apart from the rest. 'article' tends to mean 'one distinct piece': one object in a shop, one piece of writing, one numbered clause in a contract, one little grammar word in front of a noun. Each use is the same idea of 'a single, unit'.
From the word’s history
From Latin articulus 'a small joint, division, part', a diminutive of artus 'joint'. The root idea is a thing divided into separate little parts or joints — hence 'article' as one distinct, separable item, and the law sense of a document split into numbered sections.
源自拉丁文 articulus「小關節、分節、部分」，是 artus「關節」的小詞形。根本的畫面比較像「被分成一節一節、一部分一部分的東西」——所以 article 就是「一個分得出來的單件」，而合約被切成一條條編號條款的那層意思，也是從這個「分節」的畫面長出來的。
etymonline.com/word/article