“Knowledge is in itself always painless. Pain affects only the will and consists in an obstruction, impediment or frustration of it: Nonetheless, this frustration of the will, if it is to be felt as pain, must be accompanied by knowledge. That is why even physical pain is conditioned by the nerves and their connexion with the brain, so that an injury to a limb is not felt if the nerves is devitalized by chloroform. That spiritual pain is conditional upon knowledge goes without saying, and it is easy to see that it will increase with the degree of knowledge. We can thus express the whole relationship figuratively by saying that the will is the string, its frustration or impediment the the vibration of the string, knowledge the sounding-board, and pain the sound.

Now this means that not only inorganic matter but the plant too is incapable of leaving pain, however many frustrustrations its will may undergo. On the other hand, every animal, even an infusorium, suffers pain, because knowledge, however imperfect, is the true characteristic of animality. At each higher stage of animal life here is a corresponding increase in pain. In the lowest animals it is extremely slight, but even in the highest it nowhere approaches that pain which man is capable of feeling, since even the highest animals lack thought and concepts. And it is right that this capacity for pain should reach its zenith only where, by virtue of the existence of reason, there also exists the possibility of denial of the will: for otherwise it would be nothing but aimless cruelty. “

Arthur Schopenhauer, Essays and Aphorisms, Penguin Classics