oh, hard one.
sound - it's because you can talk, and you can always recall words or even air.
feelings - you mean what feelings? rapture? happiness? etc?
sights - sometimes it works, right.
tastes - you can't recall your tastes, can you?
and smell - you can't recall, but when you smell something that you smelled before, you can recall your memories, so this is the truth why it's so tricky and mystic.
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