Artificial intelligence (AI) is intelligence demonstrated by machines, as opposed to natural intelligence displayed by humans and animals. Leading textbooks define AI as the study of 'intelligent agents' that perceive their environment and take actions to achieve their goals. Popular accounts sometimes describe AI as machines that mimic human cognitive functions like learning and problem solving, but this definition is rejected by major AI researchers. Applications of AI include web search engines, recommendation systems, speech recognition, self-driving cars, automated decision-making, and games like chess. As machines become more advanced, researchers are exploring what AI can do without human help.