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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?
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.Or what if a drop of water were trickling down the trunk of the tree?The water in motion would be alive, but the motionless tree trunk would be.Modern medical advances are making it a matter of importance to decide the moment of actual death, and that is not always easy.Nevertheless, what we call "life" is sufficiently important to warrant an attempt at a definition.(Or else Earth Humans are descended from them, though this raises troublesome questions about chimpanzees.)Perhaps because of these awkward issues, Aliens with Forehead Ridges have become much less common in written SF (save for media tie-ins) than they were some decades ago.
In written SF, the KNOWN GALAXY seems increasingly to be inhabited only by Earth Humans.
Only rarely will anybody get to know one on a personal level.
TRADE with them is sporadic, and even WARFARE seems less frequent than it used to be in the GOLDEN AGE.
Nor is it used for Earth Humans who must register with the immigration service.
In general, Aliens fall into two distinct groups, REALLY ALIENS and ALIENS WITH FOREHEAD RIDGES.1) Really Aliens are truly unearthly. The occasional intelligent bear or radish may also appear, or practically anything else.
Yet what if the caterpillar were crawling over the trunk of a tree?