: Stop right there - you have not yet established that your vital spark exists.
: First establish that it exists and then establish that it takes life to
: make it.
: Right now you are begging the question.
: Until you establish THAT it exists there is little point in taking the idea
: any further. Do the ground work first.

First, there is no basis for it to exist or not. From my perspective, there is zero reason to believe it does not exist. Create life from 100% non-life, and you will establish that life can be created. There is no begging the question. Your axiom is that the "spark of life" does not exist, therefore burden of proof is on those who say it does, to change your mind. My axiom is that the "spark of life" does exist, therefore burden of proof is on those who say it doesn't. Definitions and conditions and requirements for "proof" of either stance is a much deeper conversation.

: Let's not worry about fanatics - lets assume that people will be reasonable.

You have to worry about fanatics. You always have to. If they happen to be right and you're wrong, hoooooo boy! Being "right" isn't fundamentally exclusive to "reasonable" people. As history has shown. You can't simply ignore someone's proposition because you presume them to be unreasonable.

: I do not believe that shoes can think but I'd be hard pressed to provide
: convincing proof of their lack should someone ask me to prove that they
: don't think.

Exactly. So people who believe that shoes think will always find a way to interpret the evidence to fit that belief. It entirely depends on the definition of "think", and whether it has an experimentally, and observationally disprovable set of conditions. In which case that definition can indeed be disproven. ... but then it again becomes a matter of believing and trusting the scientists who have disproven the statement. Again, until you observe it for yourself, you always trust someone else's word. For everything. "Reasonable" people are more trusting than 'conspiracy theorists' (also, those who I infer you mean by your use of the term 'fanatics')
(also 'reasonable' is an entirely subjective term)

: Normally I wouldn't worry about things that haven't been shown to exist but
: you seem to think that VS is real. How do you know? How do you know what
: it can and can not do?

How do you know it doesn't exist? How do you know? How do you know what it can and can not do?

All we know is that Life exists, and it has only ever been seen to reproduce from existing Life, and no one has created a living entity (by our own 'common understanding' of life), from non-living material. That's the observation. We can interpret those observations as evidence to support what we believe - whether there is or is not a 'spark of life'. Because it has not been observationally determined one way or another.

Well then, you may ask, what is the definition of life? If you say there is no 'spark of life', then you may as well admit that there ultimately is no "life", that machines and humans must fundamentally be the kind of entity - just a mix of atoms that can do things autonomously, even be self-sustaining, and present the capability to 'think'. I think you'd find the vast majority of human civilization would take issue with that. Alas, going back to an initial comment of mine - that's precisely the topic of conversation I love to have explored in science fiction: the nature of "Life". IS it biological? Or is it philosophical? Cultural? Religious? Supernatural? etc. What is life, and how does that play into human morals and ethics in how we treat each other?

: I have a simpler proposal. If we have no convincing evidence that X exists,
: lets just assume for now that it doesn't exist. If we ever get good
: evidence that it might exists and if we have a working method that will
: let us determine if it exists then that is the time to start taking its
: existence seriously.

If you have no convincing evidence that X does not exist (let alone any proof), let's just assume for now that it does.

No, you are starting from an assumption - you are framing the topic of the existence of X within the framework already that it does not exist, therefore anyone who says it does has the "burden of proof" - and that's exactly my point. You're starting on an assumption. Regardless of how "reasonable" the assumption is, it's still an assumption. And yes it is possible to prove a negative, if your perspective of observation is from outside the boundaries of the model in question.

Prove to me a goldfish is not in the bowl. Easy! You look at the whole of the bowl, and there is no goldfish. You might swirl a net around through the water inside to make sure there are no mirrors hiding the fish; that the fish isn't somehow transparent, or very tiny. Observation and experimentation covering the entirely of the proposed model will prove that there is no goldfish (defined by a visible golden fish-like entity) inside the bowl.

If you were an observer within the bowl, it might take a LOT longer to prove there's no goldfish in there with you. And just by yourself, you might have a problem covering the entire bowl because some could argue it's hiding from wherever you're observing, that it knows where you are and keeps out of view the entire time. You might come up with other methods to ensure that throughout the entire bowl, it is not physically possible for a goldfish to exist else it would have to have been observed. Say you spotted fish excrement; it wouldn't be proof of a fish existing in the bowl - it could be evidence used to support that position, but not proof - since you could offer another interpretation of that observation such as A) it was there long before you started observing and B) the fish could have been removed C) it could have been placed there by someone else to mislead D) etc.
* Complete observation of the entire bowl and seeing no goldfish would be the only way to irrefutably disprove the statement that "there is a goldfish in the bowl" -- or prove the statement that there is no goldfish in the bowl.
* Likewise, direct observation of the goldfish would be the only way to irrefutably prove the statement that "there is a goldfish in the bowl".

"Vital Spark"? If the conditions of its existence mean that NO life (per our understanding of life) can come into existence apart from the aid of existing life, then all you need to do to disprove that 'vital spark' theory is observe life coming into existence apart from the aid of existing life.
Simple enough.
Proving the 'Vital Spark' is much harder - it requires observation of all of life coming into existence and determining that every single entity arose from existing life.

As it stands, at this point the latter is our observation. The former is still an event that may occur. And so, whether one believes the 'vital spark' exists or not entirely depends on which perspective one adheres to. Observation has shown that the 'vital spark' is a viable belief system. Observation also supports the belief that at some point in the future humanity will observe life coming into existence from non-life, and so belief that there is no 'vital spark' is also a viable belief system. Neither have been observed (nor disproven). Both can be believed.

: Our acceptance of the existence of a thing should be proportional to the
: quality of the evidence for its existence.

I would say out acceptance tends to be, not that it should be. This is precisely the idea behind "critical thinking".
Our trust in the truth of a theory is practical insomuch as the theory holds true for us. Regardless of the quality or quantity of evidence of its existence (though more easily when there is more evidence). It's dangerous to consider an unobservable, unrepeatable theory as Absolute Truth. It may be practical and true for all intents and purposes, as long as one keeps in mind that, given it has not been directly observed, it may be uprooted at any time.
* Dogmatic assertion that an unobserved theory (an explanation of evidence) is Irrefutable Fact is a dangerous thing, even if at any point in time it's the only viable explanation. History has shown this to be true.

: There are very few things (any?) that can be absolutely proven. Therefore we
: have to weigh the evidence - that is what scientists do all the time.
: Science is an inductive discipline.

Completely agreed.

: But we shouldn't throw out the baby with the bath water and say - because we
: can't know absolutely we, therefore, know nothing. Nor should we say that
: all is opinion and all opinions are equal.

Nope. I also agree. Science moves us forward quite often because "for all intents and purposes" our explanations for what we observe are sufficient for practical purposes. This of course is in dealing with theories, as opposed to direct observation (which of course is considered irrefutable fact - again because we (as "reasonable people") trust the reports of scientists who have done and repeated the observations themselves).

: The existence of electrons is better established than the existence of your
: vital spark.

Relative comparison of non-competing theories? Irrelevant.

:: We can only choose one side of an argument that we feel bits 'fits' with our
:: own observations.
: Nonsense. Here is a clip that shows a fight between two people.
: The clip shows that there are two opposing opinions about how well the Black
: Knight did in the fight. The two opinions are not equally well supported
: and we don't have to take sides to see it.

Both sides observed the fight. They are arguing opinions based on their own definitions of a 'successful fight'. The King's success is merely that he's able to pass and continue on his way. The Knight's success is that as long as he's still alive to fight then no one is capable of passing him. Both perspectives are true, as we observe. If the King's success included killing the Knight, then he would have been wrong. But he's not. He passed the Knight. To the King, he won (and to us, because his only necessity was to pass the bridge, inclusive of the Knight, physically, and continue). To the Knight, "None shall pass" - the bridge? Or the Knight? The Knight's still alive, and fully believes he'll be able to stop the King at some point, and so believes that the King has not yet "passed", even though he'll be incapable of actually stopping the King.

To make this analogy more relevant;
If two people were judging who was the "successful" fighter in this battle, they'd have similar disagreements.
If they hadn't seen the fight at all, but both sides presented their arguments showing only clips from the video that support their premise, then they'd have to decide who they believe more, based on a pre-existing set of conditions for success - both sets of clips are just as true, they both happened, but their presentation as only a part of the whole would be an interpretation that can support the feasibility of their own explanation.

So, perhaps then you observe the entire fight and you come to your own conclusion. You may end up agreeing or disagreeing with either one of the two people, both, or neither.

Without a set of defining conditions, "who fought better" is entirely subjective. "Who won" is just as subjective without a set conditions. There need to be bounding rules in order to determine a truth. Otherwise they're just arguing opinions based within a presumed framework, or worse arguing from two perspectives with different conditions and neither can be proven right or wrong. They may both be right, or both be wrong.

To the King, the King won because he achieved his goal - to continue on his way.
To the Knight, the Knight has neither yet won nor lost, because an end condition has not yet been met. He's still alive; he's still there to fight (as ineffective as he may be), so the King has not yet won. The fight hasn't been decided. He resorts to name-calling because the King chooses not to conclude the fight.

Point:
Without a defined condition of "success" (or Truth in the matter of scientific discovery), we can only ever presume (for all intents and purposes) that any explanation (ideally, with no alternate theories) is sufficient to practically move forward, until such time as the theory is refined or refuted. Otherwise, we're just arguing opinions based within subjective frameworks, and we present those to others using evidence (interpretations of our observations) to support our position, with the hope that it's enough for others to believe, presuming of course that they trust us and our interpretation of the observations (or what we say we've observed, if we can't repeat them).

If we did not in fact observe something, or we do not have the capability of observing the entirety of the "bowl" in which it exists/happens (let alone repeat it for others to observe), then there can be no Absolute Proof.

...and wow this thread has gone way off the rails! :P