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High school students in India under the CBSE Board are taught that consciousness is a defining property of living organisms. This question lies under the topic of 'What defines living organisms?' Does that mean that if robots and other artificial intelligence have consciousness and can react to some external stimuli (just like plants), they are alive? Also, what about the patients in coma or those who are 'brain-dead', are they alive or dead?

Then, how do we define living in terms of science?

A possible answer in my opinion to the second question above could be that life begins at conception and an organism is considered alive till the point where it has no differences with the dead; i.e. a person who is 'brain-dead' would still be considered alive because he/she still isn't the same as a dead person.

So should consciousness be a defining property of living organisms?


Edit: Defining Consciousness-
In the NCERT textbook which is used to teach the CBSE high school students in India, consciousness is explained as the ability to sense the surroundings or environment and respond to the environmental stimuli which could be physical, chemical or biological.




For reference: NCERT Class 11 Biology Textbook~ Unit 1: Chapter 1, Page 5 (second paragraph) Link to book: 1http://ncert.nic.in/textbook/textbook.htm?kebo1=1-22

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    $\begingroup$ Define consciousness. Is a bacterium conscious? How about a potato? Seems like an appropriate question for the (insert choice of religion) site. $\endgroup$ – jamesqf Jan 5 at 18:34
  • $\begingroup$ @jamesqf It is mentioned in the textbook that, 'All organisms, from the prokaryotes to the most complex eukaryotes can sense and respond to environmental cues.' So this should mean that bacteria and potatoes are indeed conscious as they can react to external stimuli. I've edited the answer and added the link to the book, you can check it out. $\endgroup$ – Roger Baker Jan 6 at 15:24
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    $\begingroup$ Someone in a coma doesn't respond consciously, but their body absolutely continues to sense and respond to stimuli. A brain-dead person can still maintain body temperature, metabolize nutrients, and maintain blood pH, none of which are possible without some kind of feedback loop that requires response to the environment. Consciousness could be a necessary condition of life, but it's certainly not sufficient - my TV isn't "alive" even though it responds to the stimuli of me pressing the buttons on the remote. $\endgroup$ – Nuclear Wang Jan 6 at 15:27
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    $\begingroup$ As someone who studies consciousness...that definition of consciousness would be a very unusual one in the field. $\endgroup$ – Bryan Krause Jan 6 at 15:51
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    $\begingroup$ I have to mirror Jamesqf here there are many very different definitions of consciousness, many with a lot of baggage attached. which is why a good definition would avoid the word entirely. $\endgroup$ – John Jan 9 at 14:29
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tl;dr this is an extremely unusual definition of "consciousness" (I'd be surprised if you can find it in any standard English dictionary). If you prefer, like Humpty Dumpty in Lewis Carroll's Through the Looking Glass, to define words (in this case "consciousness") in any way you want — i.e., as above ("ability to sense the surroundings or environment and respond to the environmental stimuli ..."), then consciousness is indeed one of the defining characteristics of life.

This property is (arguably) necessary, but not sufficient, for life — things can be 'conscious' (respond to stimuli) but not be alive, but they can't be alive if they don't respond to stimuli.

This property is more typically called irritability. The Khan Academy article on "What is Life?" gives this property under the section "Response" (along with the the other properties "Organization", "Metabolism", "Homeostasis", "Growth", "Reproduction", "Evolution"). (You can also find it in lots of biology textbooks in this Google Books search.) The text you refer to uses similar criteria (growth, reproduction, metabolism, cellular organization, 'consciousness').

As Khan Academy also says, it's hard to be precise:

Living organisms have many different properties related to being alive, and it can be hard to decide on the exact set that best defines life

So, to answer your specific examples:

  • robots respond to stimuli (and, arguably, have organization and metabolisms), but they don't grow or reproduce. If they did, we might call them living organisms.
  • people who are brain-dead or in a coma still exhibit some degree of response to the environment/irritability. So do dormant seeds, spores, and other "resting stages" of organisms.

There are always edge cases; is a sterile person (or indeed one who can't reproduce because they're in a coma) "alive"? The Khan Academy web page uses a similar example, the case of mules (the infertile hybrid offspring of horses and donkeys), along with several other examples that illustrate some of the more difficult cases (dieocious/gonochoric organisms that are isolated from members of the other sex [can't reproduce, but still considered alive]; fire ["reproduces" but not organized or homeostatic]; crystals [not homeostatic, don't evolve]).

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  • $\begingroup$ And the edge cases can be even edgier. Consider HeLa cells en.wikipedia.org/wiki/HeLa or women in comas who've been raped by their attendants and become pregnant. $\endgroup$ – jamesqf Jan 8 at 4:48
  • $\begingroup$ I don't think this answers the question (which I doubt could be answered on stack exchange...). "Life" in biology typically do they have the biological machinery to replicate at the cellular level given enough resources. For almost all life, that comes down to does it have ribosomes. $\endgroup$ – James Jan 9 at 11:42
  • $\begingroup$ @jamesqf This is not a fringe case at all. HeLa cells are universally considered an in vivo ("within life") experiment. I don't understand why the rape example would be a fringe case. $\endgroup$ – James Jan 9 at 11:43
  • $\begingroup$ The title of the question is not "how do we define life?", but "should consciousness be a defining property of living organisms?" (to which the answer is, "yes, if you define 'consciousness' as ability to respond to changes in the environment) $\endgroup$ – Ben Bolker Jan 9 at 13:59
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    $\begingroup$ @BenBolker I think my overall point is that those irritability concepts are too soft, hard to define, and non-comparable between organisms without more qualifiers. I've voted to close the question. I agree that your answer is very reasonable. I just don't think any answer would completely answer this question. A more comprehensive essay on the topic of life is this Science editorial: The Seven Pillars of Life. It reframes the irritability as "adaptability" and "improvisation" I suppose. $\endgroup$ – James Jan 9 at 19:36

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