He watches them as they look on in disgust at his girlfriend – Lisa, human - and then he watches them stare in the exact same way at the cannibals in the Brecon Beacons and then he sits back and wonders why it's so easy for them to believe in alien and yet so impossible for them to believe in human.
Jack is easy. Jack doesn't know any better. He's spent all his lives fraternizing with aliens and watching humans from a distance… he can name all of the royal family members of the planet Fez but ask him how he thinks you're feeling and he's got no clue. Because he'd rather be up there, among then stars… rather emerge himself in make believe worlds full of new life and the most wonderful sights and the right kind of Doctor at his side forevermore than allow himself to remember that he's trapped down there among the old, dead race and the beauty no one takes the time to see. Jack is... wrong. Like someone's laid out the pieces of what makes people human and there's a piece missing. The piece that makes us really, truly human. The part labeled 'death'. Ianto isn't convinced that Jack knows what human is.
As for Tosh – she doesn't want to believe. Facts and science and math – easy, reliable math – all help to cover up the fact that it's so amazingly human to shun those people with the brains and the tact and the creativity so bright it makes them so different that ordinary humans can't accept. Tosh wants to believe in aliens because they gave her a life. She doesn't want to believe in human because they took it away.
Owen can't believe. He who spent so long building up his trust in humans and their science and their medicine only to watch it crumble from under his feet – the walls come crashing down so suddenly it leaves him bleeding in the sun where there is no light, because Katie is gone. The one who made him who he was. Gave him faith and strength and the sullen determination to believe in human no matter how many corpses he has to dissect or how many suicides he ticks off on the little indifferent form that takes up most of what it means to be a Doctor these days. He can't believe because there's a huge gaping hole in his life where 'human' used to be, and 'cynic' plus 'alien' can only fill up so much of it.
Gwen – Gwen wants to believe. So, so much. She looks at aliens and rift victims and labels them all so lovingly with 'human' because she thinks… she thinks it's a solution. Like being human makes everything ok again. Ianto watched her the first few weeks at Torchwood and smiled to himself because she was exactly what they needed – someone to teach them to be human again. But then the smile turned to tears as slowly she, too, forgot. It isn't easy, he supposes, doing what we do. Human fades into the background and the tiny particles of it we're exposed to every day are so repulsive that it's hard not to look at that spaceship and beg them to take you away, too. Alien is so much easier to deal with because the same rules apply to all. They're alien. End of. So easy it's almost wrong… but makes it so hard not to fall into the trap that is the loss of faith in all things human. And when Ianto looks at Gwen, that's where he thinks she's headed. He knows.
Ianto's the one who takes the cannibals coffee and tries to save the half-human woman he loved because if he doesn't believe in human… who else will?