Some sensations are too subtle to notice without a lot of training.
They tend to be heady, smell-but-not-smell affairs.
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It helps if you understand the difference between smell and other senses; it's more primal, less sophisticated than sight, goes directly.
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Smells tend to elicit memory, associations, feelings much more easily than other senses, and more automatically as well.
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Similarly, the sort of sensation I am talking about is like a mesh of associations, memories, feelings surrounding what is felt directly.