Irritation

Irritation is an observable physiological reaction to a stimulus. Assigning it the status of pain or pleasure is the perception of the being stimulated - which is not observable although it may be shared (see gate control theory of pain).

We can't say that an oyster feels pain, but we can say it does react to something irritating it, to produce something that is useful to us, and not useful to it. Pearls do not attract mates for the oyster, they attract them for us. It seems impossible to find an evolutionary advantage for the ability to produce the pearl, thus it can be explained only as a reaction to an irritation.

We can also say that an amoeba avoids a pin we poke it with, but not how much it feels this. Irritation is apparently the only universal sense shared by even single-celled creatures.

It is postulated that most such beings also feel pain, but this is a projection - empathy. Some philosophers, notably Rene Descartes, denied it entirely, even for such higher mammals as dogs or primates like monkeys - to him, intelligence was a pre-requisite to even the feeling of pain.

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