Could you please tell me what is meant by the term 'sense data'? I am not clear

Could you please tell me what is meant by the term 'sense data'? I am not clear

Could you please tell me what is meant by the term 'sense data'? I am not clear whether it refers to what we immediately perceive around us, or to images inside our heads. Or is there a third meaning? And why is it controversial?

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