I love my teacher, but I love truth more. (Aristotle quote)
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吾 is the classical Chinese equivalent of 我. You'll see it in ancient texts, quotes from Confucius, and literary writing. In modern speech it sounds archaic or deliberately formal/poetic.
grammar
In classical Chinese, 吾 was typically used as subject ('I') while 我 could be object ('me'). This distinction disappeared in modern Chinese where 我 covers both.
Bottom mouth radical - the indexing radical here. The mouth marks 吾 as a word of utterance: the one speaking, the speaker's own voice. This is the literary 'I' used in Classical Chinese (吾日三省吾身 - 'I daily examine myself three times' from the Analects).
Top 五 supplies the sound - wǔ drifting to wú with a tone shift only. 五 also lends a faint semantic flavour: 'the central one of the five,' matching 吾 as the literary first-person pronoun 'I, my own self' - the centre of one's own reckoning.
