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A Science Fiction author once posed the following question: If a race of intelligent aliens were to encounter human infants how and when would they decide that these creatures were *sentient*? In the absence of human adults on which to pattern their behavior and learn language, what characteristics would indicate that these developing mammals represent the highest evolutionary development of their home planet?
The question of sentience is not an easy one. Many folks - scientists included - would say (1) language use, (2) tool use, (3) curiosity, (4) teamwork, (5) sense of self, and (6) sense of others' needs (altruism).
The problem is that many of these characteristics are shared by animal species - dolphins and whales appear to have language, monkey and apes use tools, rats and mice will investigate new spaces and objects, ants and bees work cooperatively, all mammals have the ability to map space in self-centered vs. environment-centered coordinates, and there are many reports of family and altruistic behavior in dolphins, dogs, horses, etc.
In the 18th century, philosophers added an additional feature: to feel. Added to the previous 6, a sentient being would show all of the characteristics, and have a personal, emotional commitment to the above. Unfortunately, this definition may actually rule out some humans! (i.e. sociopaths).
Doubly unfortunate, a more modern definition of sentience invokes "consciousness." I say unfortunate, because consciousness is probably the most subjective term in all of psychology - in other words, consciousness is being aware, and we are aware because we are conscious (Velmans, M., & Schneider, S. Eds. 2007. The Blackwell Companion to Consciousness. Farthing, G. W. 1992. The Psychology of Consciousness).
As neuroscientists, we can point to enlarged Frontal Lobes as evidence of "thought" and hence of consciousness, but the question of awareness is still a troubling one - would philosophers and psychologists deny sentience to a sleeping human, unaware of their surroundings and devoid of sensory input? Or can we attribute sentience to the Internet? It uses language, communicates and even cooperates between computes, search engines are curious, the 'net uses programming tools and can tell the difference between native and foreign code - but is it aware?
I submit that we will not know until *some* creation or creature steps up to humankind and says "I think, therefore I am."