Is the universe really "fine-tuned for life"? I understand what is meant when we say the universe is "fine tuned", but I've seen it claimed it's is fine tuned "for life". Is it?
When I look at known or even speculated life in our universe, I see life clinging to a tiny crumbs separated by vast voids. Our life needs baryonic matter, only 5% of the universe. Its density is $10^{-30}  g/cm^3$, but it clumps together leaving vast voids between the clumps. 99% is in stars and black holes and nebula utterly inhospitable to known life.
That leaves the parts "fine tuned for life" to be 0.05% of the universe. The planets, asteroids, and comets seem like just debris separated by voids billions of times larger than the life-sustaining clumps preventing life from spreading.
Even that seems to be used very inefficiently. Life requires the energy of a star, but it's spewed out in all directions. We only receive a tiny fraction of it. Similarly, life requires heavier elements created in supernova, but these are also spewed out in all directions. And it took billions of years to develop a being capable of asking "is the universe fine tuned for life?"
It seems like we live on the crumbs of creation warmed by a rounding error of the heat from our star. Taking this view, the universe appears extremely inhospitable to life. It could be a lot better tuned.
Is it that the universe is fine-tuned for life? Or is it that the universe is so large and life requires such an infinitesimal fraction of the universe that it's a near guarantee were we to roll the dice again that some crumb, maybe not even baryonic matter, could develop something complex enough to ask "why is the universe fine tuned for life"? Am I just restating the weak anthropic principle?
I understand this is straying into philosophy. If that's off topic, are my  physical assumptions correct?
 A: "There must be a god" said the puddle. "Otherwise, how did the dip I sit in come to fit me so perfectly?"
There are fine tuning issues that still absorb people's attention. But fine tuning specifically for life isn't really the point.  After all, this is the only planet we are sure has life on it. So we don't really have any reason to think we know what the probability of life arising might be. Maybe life can arise in a wide range of conditions. Maybe we are "as good as it gets." Maybe we won the lottery. 
If life hadn't happened we wouldn't be here to ask about it.
We do know that nearly all of the universe is drastically inhospitable to our specific kind of life. But we also know why our specific tiny spec of the universe fits us so well. It's because we have spent many millions of years adapting to it. I'm having difficulty remembering exactly when life is thought to have arisen. I seem to recall that single cell life is vastly much older than the Cambrian explosion. But even that is about a half billion years ago. The ones that were less well adapted were less likely to have progeny. For many millions of generations.
We don't know whether some other kind of life could have adapted to another dip in the ground. Maybe if we carefully inspected at least a few other planets in other solar systems, close up and carefully enough to be sure if there was life, maybe we would have a better notion.
A: The only unquestionable truth (dare I say, tautology) is that the universe MUST allow life, otherwise the universe in question cannot be observed or experienced. This is the philosophical basis for the Anthropic principle (AP). An observable universe might be otherwise arbitrary, unless constrained by other universal principles. But I am not aware of any other principle that is as universal as the AP.
There are many variations and refinements on the principle based on Bayesian analysis. We can further demand that an observable universe must not only allow for any kind of life, but intelligent life, otherwise we would not be here questioning the nature of natural laws
The principle implies that you can use it to argue for unnatural conditions of the universe. The limitation is that in order to keep the argument formal, you need also to provide convincing evidence that entropically favorable conditions are usually hostile to intelligent life, which is something that is impossible without a deeper understanding of the fundamental theories from which our known theories emerge
A: The argument that the universe is "fine tuned for life" is structurally identical to your argument that the visible universe is largely inhospitable to life.
The idea is that the visible universe is just a small part of what exists, a part in which life happens to be possible. The claim that the "universe" (meaning visible universe) is fine tuned for life is precisely a claim that life is impossible in most of the "multiverse" (meaning all that exists). If the whole visible universe was teeming with life, that would be a strong argument against fine tuning.
If someone argues that God chose the parameters of the Standard Model to make life possible, then your rebuttal is reasonable: if the whole point is to support life then why make all those parts that don't? But that isn't the claim that physicists are making when they suggest that the parameters are anthropically determined.
A: Your view is the view of Stephen Weinberg.
Personally, I think it is over emphasising the vast distances between inter-stellar space. Not that they aren't vast but because even this earth is already vast. It's much too large for one person to explore. In fact, never mind the entire earth, London is too vast to explore for one man. Or even just one corner of it. Or perhaps just one street. What does it mean, after all to know just one street? Is it to know every brick? To know every stone? To know every grain? This is why William Blake said "to see a world in a grain of sand". Most of course don't.
To say that the universe is "fine-tuned for life", the strong anthropic principle, brings back the principle of telos (purpose) back into physics. In fact, the original metaphysics of physics, that is by Aristotle, was predicated on a telos. This, I expect remained the case during the Aristotelian period of European science & philosophy until Newton, after which there was a rapid move, relatively speaking, to a world, that is a universe, without a telos. Newton would have been dismayed as he was a devout Christian. That move was predicated on an over-turning of Aristotelian philosophy that supposedly Newton heralded. But of course we know that Newton would not have supported such a view. Newton would not have read his work as a break of Aristotelian philosophy, but a continuation of it. After all, his critique of his own theory of gravity was through the Aristotelian critique of action at a distance. To recall how astute his critique was, it's worth recalling this is exactly what Einstein accomplished by discovering GR, a local theory of gravity where there was no action at a distance.
What is it that Newton understood that Weinberg doesn't?
Personally, I think the Aristotelean notion of a telos to the universe has a great deal merit to it, but our understanding of this sense is still early and pretty meager. Probably the best exposition of this idea was Lovelock's notion of Gaia, that earthly life interacts with the inorganic environment to regulate the conditions of life on this earth.
