— From The Master Algorithm – How the Quest for the ultimate learning machine will remake our world by Pedro Domingos P. 67
A conjunctive concept is what Tolstoy had in mind when he wrote the opening sentence of Anna Karenina: “All happy families are alike; each unhappy family is unhappy in its own way.” The same is true of individuals. To be happy, you need health, love, friends, money, a job you like, and so on. Take any of these away and misery ensues.
In machine learning, examples of a concept are called positive examples, and counterexamples are called negative examples. If you’re trying to learn to recognize cats in images, images of cats are positive examples and images of dogs are negative ones. If you compiled a database of families from the world’s literature, the Karenins would be a negative example of a happy family, and there would be precious few positive examples.”