Xkcd acceptable dating ages

However, Aliens with Forehead Ridges continue to thrive in Hollywood Scifi. In casual encounters with the material universe, we rarely feel any difficulty here, since we usually deal with things that are clearly alive, such as a dog or a rattlesnake; or with things that are clearly nonalive, such as a brick or a typewriter.This is for an obvious reason: the audience wants aliens of One of the first ways in which we learn to classify objects is into two groups: 1. Nevertheless, the task of defining "life" is both difficult and subtle; something that at once becomes evident if we stop to think. The caterpillar is alive, but the rock is not; as you guess at once, since the caterpillar is moving and the rock is not.Is it easy to tell a live seed from a dead seed, or either from a grain of sand?For that matter, is it always easy to tell whether a man is merely unconscious or quite dead?Exosemanticists have their work cut out understanding them, and exopsychologists in figuring out what they're all about.

Implies human-level cognitive and linguistic ability but not necessarily tool use.

It would be expecting much of anyone to guess that an oyster were alive if he came across one (for the first time) with a closed shell.

Could a glance at a clump of trees in midwinter, when all are standing leafless, easily distinguish those which are alive and will bear leaves in the spring from those which are dead and will not?

Yet what if the caterpillar were crawling over the trunk of a tree?

The trunk isn't moving, yet it is as alive as the caterpillar.

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