Most generally, the capacity to deal flexibly and effectively with practical and theoretical problems. Since people’s capacities to do this vary with the problem, it may be doubted whether there is a useful level of abstraction at which one thing, intelligence, can be thought of as equally manifested in whatever logical, theoretical, practical, mathematical, linguistic, etc. successes we achieve. Nor is there much confidence left that intelligence tests measure any such general capacity, as opposed to measuring the subject’s capacity to take intelligence tests, often of very specific and culturally peculiar kinds. For the question whether humans alone possess intelligence, see animal thought, instinct.