From Fernando Pessoa’s The Book of Disquiet:
One of my constant preoccupations is trying to understand how it is that other people exist, how it is that there are souls other than mine and consciousnesses not my own, which, because it is a consciousness, seems to me unique. I understand perfectly that the man before me uttering words similar to mine and making the same gestures I make, or could make, is in some way my fellow creature. However, I feel just the same about the people in illustrations I dream up, about the characters I see in novels or the dramatis personae on the stage who speak through the actors representing them.
I suppose no one truly admits the existence of another person. One might concede that the other person is alive and feels and thinks like oneself, but there will always be an element of difference, a perceptible discrepancy, that one cannot quite put one's finger on. There are figures from times past, fantasy-images in books that seem more real to us than these specimens of indifference-made-flesh who speak to us across the counters of bars, or catch our eye in trams, or brush past us in the empty randomness of the streets. The others are just part of the landscape for us, usually the invisible landscape of the familiar.
I feel closer ties and more intimate bonds with certain characters in books, with certain images I've seen in engravings, that with many supposedly real people, with that metaphysical absurdity known as 'flesh and blood'. In fact 'flesh and blood' describes them very well: they resemble cuts of meat laid on the butcher's marble slab, dead creatures bleeding as though still alive, the sirloin steaks and cutlets of Fate.
I'm not ashamed to feel this way because I know it's how everyone feels. The lack of respect between men, the indifference that allows them to kill others without compunction (as murderers do) or without thinking (as soldiers do), comes from the fact that no one pays due attention to the apparently abstruse idea that other people have souls too.
I wish Pessoa, continuing down the “cuts of meat” line of thinking, had recognized that this ignorance is also what allows us to do what we do to animals — by assuming that your counterparty has no soul, anything is permissible.
One solution is trying to live by the golden rule more. Yes, we have no insight into the consciousness of others, but we also know that they, too, have no insight into our own minds. If we’d prefer they nonetheless treat us as an “end in itself,” we ought to extend the same privilege to them.