Like life and beauty, intelligence is a surprisingly difficult quality to define. However, just as the search for extraterrestrial life demands careful consideration of what life is and therefore what 'signatures' should be sought in the effort to reveal its presence (see extraterrestrial life, detection), so the quest for intelligence beyond the Earth calls for a close examination of the nature of intelligence. The scale of this problem is made clear by the fact that we have trouble in defining and quantifying intelligence even within our own species. So-called "intelligence tests" tend to measure only a very narrow band of mathematical, verbal, and vi suo-spatial skills. Creativity, intuition, imagination, artistry, and many other abilities must be taken into account in any true, multidimensional view of what intelligence means. Clearly, there are different ways in which high degrees of intelligence can be manifested. It may be hard to decide who was the greater scientific genius, Newton or Einstein, but it is probably meaningless to try to calibrate the relative intelligence of, for example, Einstein, Shakespeare, Gandhi, and Bach.

Moving beyond Homo sapiens, to some of our fellow terrestrial species, the difficulties in assessing the presence and level of intelligence are compounded. Whatever we take intelligence to mean, it evidently forms a continuum. An amoeba is not intelligent by our standards, yet it has intelligence to a limited degree. A snake, by anyone's measure, is more intelligent than an amoeba, and a dog brighter than a snake. Brain-to-body mass ratio and neural complexity are obviously important correlates of intelligence. Thus we find that cetaceans and cephalopods, which have big, complex nervous systems, display apparently intelligent behavior. Yet these creatures live and communicate in such non-human ways that we begin to see in them the possible diversity that intelligence may take in extraterrestrial environments. There is the possibility, too, of significant levels of intelligence based, not on large, single brains, but on a more distributed processing arrangement, such as exists in some insect colonies (see hive intelligence) and perhaps even in entire ecosystems (see Gaia Hypothesis). Finally, intelligence is almost certainly not exclusive to organic systems. It may be that, within a few decades, artificial intelligence will rival or exceed our own.