A comprehensive life philosophy is a big concept, so what follows is just a brief summary of the ideas that drive both my work and personal life. It deviates significantly (and intentionally) from "common sense" ethics, but is heavily influenced by modern scientific philosophy. In particular, the ideas of David Deutsch in The Beginning of Infinity resonate with me (though the arrogant tone of the book does not). My pursuit of a comprehensive philosophy is ongoing, and I have much left to read. However, I think its current state is sufficiently sophisticated to justify sharing and discussing with others.
The quality of a conjecture or hypothesis can be broken down into its intrinsic and extrinsic quality. I'll use the term conjecture when speaking of the first and hypothesis when speaking of the second, but both refer to the same thing. The intrinsic quality is a measure of the explanatory power of the conjecture. Arbitrary, circular, or inconsistent conjectures are poor explanations and should not be admitted (this criterion is taken directly from David Deutsch). Note that arbitrarity, circularity, and inconsistency are not easy to detect in highly complex theories, so this criterion is far from trivial. The extrinsic quality of the hypothesis is how well the hypothesis describes reality. The predictions of a hypothesis should be tested against real observations (i.e. the Scientific Method). Notably, the observations should not be available when the hypothesis is formulated.
Intuitively, these characteristics come from Mathematics and Science, respectively. They serve the independent, complementary goals of making a conjecture powerful and true, again respectively. Seeking to justify my choice of epistemic assumptions unfortunately exposes a deep flaw: it is justified by itself (circular), which means it is also arbitrary and inconsistent. The best reason I have to believe its truth is based on the evidence of millennia of human progress, which, of course, was available to me when I (and others) formulated it. However, these are not reasons to dismiss the theory: all epistemological theories are open to this criticism, known as the Regress Problem in Philosophy and its cousin the Incompleteness Theorem in Mathematics. Note that only epistemological theories make this problem unavoidable: the judgment criteria listed above can and should be used to dismiss non-epistemological theories.
Still, we're in a bit of a mess if we're making exceptions right off the bat. The best way to get around the regress problem is to accept that no theory can ever be perfect: all we can do is to continually improve upon our current one. This requires openness to new ideas at all levels of understanding, which leads directly to the idea that nothing is above criticism, including the notion that nothing is above criticism. Using the three propositions in bold above, with the last being the most important, we establish a mechanism for continual, self-reinforcing progress, or what David Deutsch calls "a beginning of infinity".
I initially wanted to call this section "Conclusions", but it felt like too final a word for the continually-evolving nature of my personal philosophy. As I said above, one of my deepest values is intellectual humility. None of these current theories is beyond reproach, though I am more confident in some than others.
God and Religion
I don't think there is a God. The idea of God or gods fails spectacularly on all the aforementioned measures of a good idea. Intrinsically, it is totally arbitrary, circular by nature ("there is a God because it was always there"), and prone to self- contradiction (especially in those religions with a large corpus of accumulated scripture). Extrinsically, the evidence for the truth of religious belief is denied necessity by its proponents, who claim that faith (which is to say belief without evidence) is not only necessary but desirable. These premises make for an ideology that is extraordinarily difficult to shake (not to mention the social context that reinforces it further). Indeed, my gradual conversion from Christianity to Atheism was a very distressing period of my life. By holding themselves above criticism, religions fail the most important test of all.
So, I reject religion on the aforementioned grounds. Though I strive to remain open to persuasion, this is a particular case where I have invested significant time and research into searching for anything that could convince me otherwise. I am skeptical that something yet remains, though, of course, I'm not totally closed to the idea.
It is worth noting here that I'm not a militant atheist. I think there's merit to the hypothesis that eliminating religion from modern society will remove the fetters that have kept human creativity stumbling rather than bounding forward. However, sudden change typically means greater polarization, which increases conflict and potentially violence. This is a risk that should be considered when pushing against the establishment, especially when trends seem to point toward secularism in the long run anyway.
Nothing "happens for a reason". Often cited as one of the fundamental reasons for the abundance of religion, the desire for an existential purpose is an understandable one. I think it is borne of two elements, one biological and the other rational.
Evolutionary psychology brings us the idea that humans gained an advantage by being better able to understand the world than their less-enlightened competitors. This understanding came from an instinctive curiosity, a desire to find the reason things happened the way they do. This desire could not have been instilled through rational thought, since it came before rational thought existed. Therefore, our brains are wired to seek an underlying purpose for anything and everything we encounter. Once we became intelligent enough to conceive of our own existence, it was inevitable that we would seek an underlying purpose for that too. However, this is merely an instinct that is being applied outside its useful scope. Cravings for purpose, like cravings for sex and food, can lead to highly undesirable outcomes.
The rational aspect of the search for purpose is the one that gives it legs in this era of enlightenment. Namely, it is the juxtaposition of a larger purpose with nihilism: "Nothing happens for a reason, so everything is pointless." Faced with a choice between these two alternatives, the vast majority of people understandably choose the former. Indeed, any who wholeheartedly embraced nihilism would have a dramatically-reduced probability of propagating their genes and memes, either due to suicide or despondence. I am no nihilist, but as I said, I also do not believe in purpose. Instead, I think the choice is merely a false dichotomy brought about by a cognitive illusion for which I do not know the name (please let me know if you do). I'll call it an illusion of scale by abstraction.
The human mind is only able to retain and reason about a certain degree of complexity. In order to reason about highly-complex systems, we use approximations called abstractions, which package up the complexity of a system into a more simple representation with rules that approximate the behavior of the underlying system. For example, the idea of a government is an abstraction of the actual behavior of large numbers of people organized in a mutually-beneficial way. Similarly, a star is an abstraction for the atoms collected together in such a way that gravity causes fusion to occur, and a galaxy for the collections of hundreds of billions of stars that can spontaneously form. Abstraction is an invaluable tool in the repertoire of reason, as it allows us to think about concepts that vastly exceed the inherent capability of our brains. Unfortunately, it also comes with a cost: reasoning with abstractions requires us to ignore the true complexity of the system we are trying to analyze. Thus, it becomes easy to fall into the trap of thinking that everything we can think of has about the same degree of complexity. This tendency can fool people into thinking that a lack of an overriding purpose to the universe somehow nullifies its significance. This idea misses the fact that "the universe" is almost entirely incomprehensible to the human mind. It is a structure containing somewhere around 10^22 stars, each of which is about 10^9 meters in diameter. For a species that evolved to think about objects ranging from 10^-3 to 10^3 meters in size, these scales are meaningless outside of the helpful abstractions we use to think about them. Similar discrepancies between the limits of existence and of our consciousness exist in many other respects: time, energy, mass, social networks, financial constructs, artistic endeavors. Losing track of the sheer enormity of our universe is all too easy. Losing sight of the fact that greater scale not only increases the number of actors in a system, but also the complexity of their interactions (by the square of the multiple for observant graph theorists), is even easier.
The truth is that we are currently (and likely, forever will be) incapable of comprehending the potential inherent in a universe with intelligent life. Trying to ascribe an ethno-centric purpose to such a vast structure is not just naive. It is arrogant, and would yield a result far less appealing than the truth. Ironically, trying to apply a purpose external to humanity also trivializes the importance of humanity itself to the future of the universe. Intelligence (or "explanatory knowledge" in the words of Deutsch) plays a critically-important role in this theory, making us (or rather, all intelligent life) the most significant emergent phenomenon in the universe.
Ethics and Morality
So, then, if not from religion, where does ethics come from? Absent a purpose, how can we decide what we, as a species or individuals, should do?
Well, until recently, I was a Two-level Utilitarian , which contends that the goal of ethics is to maximize the happiness of people. It does so by laying out general rules that people should follow most of the time, but leaving room for exceptions when a decision that breaks these rules would yield a superior outcome. This theory works for most ethical dilemmas, giving good results for situations like common crime and wars. Unfortunately, it has two significant problems. Firstly, it suffers from the same circularity as our epistemological theory: the choice of "people's happiness" is an arbitrary one. Secondly, its definition exposes it to all sorts of moral gray areas where it has no say. What defines a "person"? What is their "happiness"? The first question causes utilitarianism to stumble over questions like the ethics of animal treatment, abortion, and mentally retarded humans. The second one trips on the question of drugs and virtual realities: if we could create technology that allowed people to be happy all the time at the cost of species-wide stagnation, should we?
These are tremendously difficult questions to answer, even though most people do have an opinion. The problem is that there is no agreed-upon goal to aim towards for any sort of consensus. The utilitarian goal stands on too weak a foundation to provide a basis for all ethical questions.
A goal that solves these problems to pursue is knowledge (again, I'm building off of Deutsch here). What follows is pretty abstract and certainly needs more exploration (Deutsch's treatment is also disappointingly superficial), but it is a powerful idea even before such development. The thinking is as follows. Knowledge is information that describes reality. It is created by intelligent beings. It is the ultimate enabler: anything that is physically possible can be accomplished with the right knowledge. It leads to the creation of interesting and beautiful phenomena: life is the instantiation of the knowledge encoded into DNA and the biological machinery that replicates it (although be aware that this knowledge, created through the slow process of natural selection, is inherently less powerful than the explanatory knowledge people are able to create via conjecture, criticism, and experimentation). Knowledge is a fundamentally important emergent phenomenon in the universe, and the pursuit of it is a goal worthy of a central place in our theory of ethics. So, I think a theory of ethics superior to utilitarianism is one that, rather than maximizing knowledge, maximizes the collective knowledge of society over time.
A few examples can help clarify what I mean. As I said, life is a tremendous store of knowledge. This is clear from the fact that, despite our considerable understanding of the laws that govern stars light years away, we understand relatively little of the mechanisms that govern our own bodies. Thus, this theory has already given life a high valuation; considerably higher than the simple constructs that are rocks or rivers. The more complex the life form, the more valuable it is, matching our intuition that the life of a dog or monkey is worth more than the life of a tree or jellyfish. Intelligent life, however, gets a special place because of our ability to create knowledge far faster than any other life form. We are more important than they are, though not infinitely more so.
Room for Improvement
The above is incomplete. It stays pretty far away from practical topics like economics, politics, or personal relationships. In part, the incompleteness is pragmatic: I can't take the time to write a treatise on the entirety of my life philosophy. A larger part is the incomplete nature of my life philosophy. It's a work in progress, and I intend to add more as I develop it more. However, I think what I have so far forms a strong foundation to build on. It also provides a strong motivation to pursue a successful, fulfilling career: the creation of knowledge as the ultimate good. For now, that'll have to do.