This one noun covers two things that look unrelated: money taken off a total, and a conclusion drawn from what you already know. The link is in the older picture behind the word — leading something down or away from a bigger thing. Lead an amount away from a total and you have a deduction on a ; lead a conclusion down from facts you already hold and you have Sherlock Holmes.
From the word’s history
Deduction comes from Latin deducere 'to lead or bring away or down' (de- 'down' + ducere 'to lead'). By the mid-15th century it meant 'a taking away, an amount subtracted'; around the 1520s it was taken up in logic for 'derivation as a result from a known principle, an inference'. Both senses keep the same picture of leading something down from a larger whole.
deduction 來自拉丁文 deducere「往下帶、帶走」（de-「向下」＋ ducere「帶領」）。十五世紀中葉，它指的是「拿走、被減去的數額」；到 1520 年代前後，邏輯學借用它來指「從已知原則推導出的結論」。兩個意思保留的是同一個畫面：從一個更大的整體那裡，把某樣東西引下來。
Online Etymology Dictionary (etymonline.com), entry "deduction"
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原始詞典資料5 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。
deductionnoun/dɪˈdʌkʃən/
- 1
That which is deducted; that which is subtracted or removed.
- Near-synonyms: extract, reduction; see also Thesaurus:decrement
- 2
That which is deducted; that which is subtracted or removed.
- standard deduction
- You might want to donate the old junk and just take the deduction.
- 3
businessfinancelawThat which is deducted; that which is subtracted or removed.
- 4
human-scienceslogicmathematicsA process of reasoning that moves from the general to the specific, in which a conclusion follows necessarily from the premises presented, so that the conclusion cannot be false if the premises are true.
- He arrived at the deduction that the butler didn't do it.
- An unkinder deduction, considering only the Select Committee's report and discounting rumours of wayward conclusions by the Stedeford Group, would be that if one inquisition doesn't produce the desired answers, the Minister's policy is "try, try again"—or should one say "fight, fight, fight again".
- 5
human-scienceslogicmathematicsA process of reasoning that moves from the general to the specific, in which a conclusion follows necessarily from the premises presented, so that the conclusion cannot be false if the premises are true.
- Through his powers of deduction, he realized that the plan would never work.
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。