"Cats, dogs, and some I mean, birds, many species of mammals, they also have the sort of potential to show affection firstly because of the biological factor."

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And then there were cats, thought Dog. He'd surprised the huge ginger cat from next door and had attempted to reduce it to cowering jelly by means of the usual glowing stare and deep-throated growl, which had always worked on the damned in the past. This time they had earned him a whack on the nose that had made his eyes water. Cats, Dog considered, were clearly a lot tougher than lost souls. He was looking forward to a further cat experiment, which he planned would consist of jumping around and yapping excitedly at it. It was a long shot, but it just might work.

— Terry Pratchett