Think of things that have been scattered — sheets of paper across a desk, poems printed in twenty different magazines — being picked up one by one and stacked in one place. Nothing new is added; what was spread out is simply brought together. Both uses of collected can be seen as that same stack: once as pages in a book, once as a person's thoughts inside their head.
From the word’s history
Wiktionary records collected as collect + -ed, that is, 'having been gathered'. That fits both modern uses: writings gathered into one book, and a person whose thoughts are gathered rather than scattered.
Wiktionary 記的是 collect（收集）加上 -ed，也就是「已經被收攏」。這條線索剛好對上今天的兩個用法：文章被收成一本，心思被收回來所以人很鎮定。
Wiktionary (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1078119932), etymology_text: 'From collect + -ed.'
原始詞典資料3 筆記錄
以下是外部詞典記錄的用法，摘錄並精簡格式，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
collectedadj/kəˈlɛktɪd/
- 1
Gathered together.
- the collected works of Charles Dickens
- 2
Cool‐headed, emotionally stable, in focus.
- He stayed collected throughout the ordeal.
- In an instant Francesca would become perfectly collected—every past event would stand out singularly clear, and she would turn, take one look at Guido, and then breathe again.
這個拼法也被記錄為下列字的變化形：
摘錄自 英文維基詞典 English Wiktionary，已精簡格式；依 CC BY-SA 4.0 授權使用，改動後的內容同樣以此授權提供。