While I was out for a walk today, something occurred to me that is pretty obvious, yet I hadn't thought of it quite this way before -- namely, that there can never be any objective answer to the question, "What is life?"
By "What is life?", I don't mean the strictly biological question of what physical processes are necessary to maintain the existence of an organism. Instead, I mean: "What is the nature, meaning, purpose, or significance of my life, or of the lives of others?"
Life, in this sense, is a set of subjective experiences. Everything we perceive, remember, or imagine is subjective -- a thought, image, sensation, etc. in our field of consciousness. This is not to say that there is no objective component to our experiences. There may well be. But we cannot access this objective component directly. What we know directly is only our personal experience, which is necessarily subjective.
Nor are we entitled to assume that the objective component, if any, is identical to our subjective experience of it. We simply don't know. A case can be made that our method of experiencing reality has developed in such a way as to simplify the world, and that our perceptions may no more resemble the underlying structure of reality than the icons on a computer desktop resemble the underlying applications they represent. This argument has been developed in some detail by Donald D. Hoffman, a professor of cognitive sciences at UCI, in his paper "The Interface Theory of Perception" (PDF).
In any event, all that we directly know consists of our subjective experience.
Now, suppose someone were to ask whether the formula "E equals mc squared" tastes sweet or sour. The obvious answer is that it has no taste, because mathematical formulas do not belong to the category of things we can taste. The question is a category error.
Well, subjective experiences are not part of the category of things that are objective. Therefore, if life is a set of subjective experiences, then looking for an objective answer to the question "What is life?" is also a category error. Or so it seems to me.
Objective methods of proof are possible only in regard to things that can be measured objectively. For practical purposes, we generally agree on what those things are. We can agree that there are objectively ten laboratory rats in a cage because we all agree on the existence of laboratory rats and the reliability of our sense perceptions. In fact, we are actually agreeing only that we all perceive ten rats, but as a practical matter, we assume that our perceptions accord with some underlying reality. To this extent objectivity is possible.
But when it comes to purely subjective experiences, how can there be any objective validation or proof? Without such validation, these experiences are doomed to be considered scientifically unproven, and some people will reject them for this reason. But in this case, "scientifically unproven" merely means that the particular method of science is not applicable to these experiences. There is no ground for saying that the experiences are unreal or unimportant just because they do not happen to fit the particular methodology that science employs.
Imagine if science were to "prove" that there is no such thing as love -- that love is purely a chemical reaction with no moral or spiritual significance. Would anyone be tempted to give up on love, to divorce his spouse or abandon his children, to deny his own subjective experience of love, merely because some presumed authority had ruled against it? But of course science could not "prove" such a proposition in the first place. The most it could prove is that there is some chemical state that is correlated with the subjective experience of love. Science as such cannot go beyond that point. Some individual scientist might presume to do so, but then he would not be speaking for "science" as such. He would be expounding his own private philosophy.
When I came back from my walk, I looked up an essay by William James called "Is Life Worth Living?", which can be read in full here. I had a vague recollection that there was something in this essay relevant to the subject at hand.
What follows is the final section of the essay, considerably compressed, and with a couple of the longer paragraphs broken up for easier reading. (It is worth reading the whole thing at the link provided.)
Now, I wish to make you feel, if I can in the short remainder of this hour, that we have a right to believe the physical order to be only a partial order; that we have a right to supplement it by an unseen spiritual order which we assume on trust, if only thereby life may seem to us better worth living again. But as such a trust will seem to some of you sadly mystical and execrably unscientific, I must first say a word or two to weaken the veto which you may consider that science opposes to our act.
There is included in human nature an ingrained naturalism and materialism of mind which can only admit facts that are actually tangible. Of this sort of mind the entity called 'science' is the idol. Fondness for the word 'scientist' is one of the notes by which you may know its votaries; and its short way of killing any opinion that it disbelieves in is to call it 'unscientific.' It must be granted that there is no slight excuse for this. Science has made such glorious leaps in the last three hundred years, and extended our knowledge of nature so enormously both in general and in detail; men of science, moreover, have as a class displayed such admirable virtues, -- that it is no wonder if the worshippers of science lose their head....
[But science is still in its infancy, and therefore] our science is a drop, our ignorance a sea. Whatever else be certain, this at least is certain, -- that the world of our present natural knowledge is enveloped in a larger world of some sort of whose residual properties we at present can frame no positive idea.
Agnostic positivism, of course, admits this principle theoretically in the most cordial terms, but insists that we must not turn it to any practical use. We have no right, this doctrine tells us, to dream dreams, or suppose anything about the unseen part of the universe, merely because to do so may be for what we are pleased to call our highest interests. We must always wait for sensible evidence for our beliefs; and where such evidence is inaccessible we must frame no hypotheses whatever.
Of course this is a safe enough position in abstracto. If a thinker had no stake in the unknown, no vital needs, to live or languish according to what the unseen world contained, a philosophic neutrality and refusal to believe either one way or the other would be his wisest cue. But, unfortunately, neutrality is not only inwardly difficult, it is also outwardly unrealizable, where our relations to an alternative are practical and vital. This is because, as the psychologists tell us, belief and doubt are living attitudes, and involve conduct on our part. Our only way, for example, of doubting, or refusing to believe, that a certain thing is, is continuing to act as if it were not. If, for instance, I refuse to believe that the room is getting cold, I leave the windows open and light no fire just as if it still were warm. If I doubt that you are worthy of my confidence, I keep you uninformed of all my secrets just as if you were unworthy of the same. If I doubt the need of insuring my house, I leave it uninsured as much as if I believed there were no need.
And so if I must not believe that the world is divine, I can only express that refusal by declining ever to act distinctively as if it were so, which can only mean acting on certain critical occasions as if it were not so, or in an irreligious way. There are, you see, inevitable occasions in life when inaction is a kind of action, and must count as action, and when not to be for is to be practically against; and in all such cases strict and consistent neutrality is an unattainable thing.
And, after all, is not this duty of neutrality where only our inner interests would lead us to believe, the most ridiculous of commands? Is it not sheer dogmatic folly to say that our inner interests can have no real connection with the forces that the hidden world may contain? ... Science as such assuredly has no authority, for she can only say what is, not what is not; and the agnostic "thou shalt not believe without coercive sensible evidence" is simply an expression (free to any one to make) of private personal appetite for evidence of a certain peculiar kind....
It is a fact of human nature, that men can live and die by the help of a sort of faith that goes without a single dogma or definition. The bare assurance that this natural order is not ultimate but a mere sign or vision, the external staging of a many-storied universe, in which spiritual forces have the last word and are eternal, -- this bare assurance is to such men enough to make life seem worth living in spite of every contrary presumption suggested by its circumstances on the natural plane....
Probably to almost every one of us here the most adverse life would seem well worth living, if we only could be certain that our bravery and patience with it were terminating and eventuating and bearing fruit somewhere in an unseen spiritual world. But granting we are not certain, does it then follow that a bare trust in such a world is a fool's paradise and lubberland, or rather that it is a living attitude in which we are free to indulge? Well, we are free to trust at our own risks anything that is not impossible, and that can bring analogies to bear in its behalf....
[James analogizes the life of a domestic animal, who cannot conceive of the larger sphere of human relations around him, to the life of a human being, who cannot fathom the possible larger sphere of divine activity around him.] In the dog's life we see the world invisible to him because we live in both worlds. In human life, although we only see our world, and his within it, yet encompassing both these worlds a still wider world may be there, as unseen by us as our world is by him; and to believe in that world may be the most essential function that our lives in this world have to perform.
But "may be! may be!" one now hears the positivist contemptuously exclaim; "what use can a scientific life have for maybes?" Well, I reply, the 'scientific' life itself has much to do with maybes, and human life at large has everything to do with them. So far as man stands for anything, and is productive or originative at all, his entire vital function may be said to have to deal with maybes. Not a victory is gained, not a deed of faithfulness or courage is done, except upon a maybe; not a service, not a sally of generosity, not a scientific exploration or experiment or text-book, that may not be a mistake. It is only by risking our persons from one hour to another that we live at all.
And often enough our faith beforehand in an uncertified result is the only thing that makes the result come true. Suppose, for instance, that you are climbing a mountain, and have worked yourself into a position from which the only escape is by a terrible leap. Have faith that you can successfully make it, and your feet are nerved to its accomplishment. But mistrust yourself, and think of all the sweet things you have heard the scientists say of maybes, and you will hesitate so long that, at last, all unstrung and trembling, and launching yourself in a moment of despair, you roll in the abyss.
In such a case (and it belongs to an enormous class), the part of wisdom as well as of courage is to believe what is in the line of your needs, for only by such belief is the need fulfilled. Refuse to believe, and you shall indeed be right, for you shall irretrievably perish. But believe, and again you shall be right, for you shall save yourself. You make one or the other of two possible universes true by your trust or mistrust, -- both universes having been only maybes, in this particular, before you contributed your act....
Suppose, however thickly evils crowd upon you, that your unconquerable subjectivity proves to be their match, and that you find a more wonderful joy than any passive pleasure can bring in trusting ever in the larger whole. Have you not now made life worth living on these terms? What sort of a thing would life really be, with your qualities ready for a tussle with it, if it only brought fair weather and gave these higher faculties of yours no scope? ....
Here is our deepest organ of communication with the nature of things; and compared with these concrete movements of our soul all abstract statements and scientific arguments -- the veto, for example, which the strict positivist pronounces upon our faith -- sound to us like mere chatterings of the teeth. For here possibilities, not finished facts, are the realities with which we have actively to deal....
Believe that life is worth living, and your belief will help create the fact. The 'scientific proof' that you are right may not be clear before the day of judgment (or some stage of being which that expression may serve to symbolize) is reached. But the faithful fighters of this hour, or the beings that then and there will represent them, may then turn to the faint-hearted, who here decline to go on, with words like those with which Henry IV. greeted the tardy Crillon after a great victory had been gained: "Hang yourself, brave Crillon! we fought at Arques, and you were not there."