but why do we use "felt" instead of "feel"?
human needs are states of felt deprivation.
what does the "felt" mean?
past tense of "to feel"
When you don't have a direct object, assume that the universal "one" is being referred to.
Rephrase the sentence using one and it makes sense.
"human needs are states of deprivation one felt"
Actually I take my earlier reply back. I think this sentence has to do with the verb's past participle being used almost as an adverb, similar to how a gerund is used in place of a noun.
felt emotion could be sought emotion, thought emotion, done emotion, passed emotion.... see?
In other words, in this case they are not treating "felt" as a verb, but almost as an adjective. Replace "felt" with "purple" and you'll see what I mean. They're modifying the noun "emotion", not the verb "to feel".