Paul was master of ceremonies at the 2012 Reason Rally (i.e. he's sort of a new atheist activist), and previously appeared on Danny Lobell's Modern Day Philosophers Podcast, where he kept mentioning existentialism, so I thought it only fitting to get him to read an existentialist who, while not a member of a religious sect, thought that science alone was inadequate to provide a foundational world view for living.
Science, according to Jaspers, who you'll want to keep in mind started as a scientist himself, investigates particular domains with practical purposes in mind. As psychologist, he studied mental illness in order to treat it. But when studying people, i.e. turning a self into an object for study, there's always some part of the self that escapes objectification, something we can't get ahold of. An obvious aspect of this is your own freedom: by looking at yourself as biology or psychology or part of physics or chemistry or sociology, you're not going to discover your own experience of freedom. Yet to live fully and effectively, you have to see yourself as free. Doing otherwise would make you a slug; it would undermine your potential. Embracing your possibilities, throwing yourself into committed action (and taking a philosophical position is included in "action" here), is what Jaspers calls Existenz. Though he later, when Sartre popularized the term, denied being an "existentialist," it's clear that he was one of the founders of the movement, an older contemporary of Heidegger's who influenced the latter's thought, who in turn influenced Sartre.
So science is a collection of facts, and doesn't amount to a "world view," which is something that as human beings we need. Even the scientist has such a world view, i.e. one that requires only believing propositions supported by empirical evidence, rejecting superstition, etc. Jaspers would agree with the rigor involved with this approach, but would point out that the approach itself is not given as a conclusion of scientific investigation. It's something that we inherit from a historical tradition, founded of course in part on its success in fostering scientific progress, but as an ethos it goes beyond the limited, hypothetical character of specific scientific findings. By "hypothetical" here I mean that any given scientific investigation has to provisionally assume a whole body of prior knowledge, e.g. that the instruments in question work, that the physical world is real, or within chemistry, that molecules and atoms exist, etc. etc. etc. So while you may not have any reason to doubt these assumptions, you have to keep in mind that what you've really proven is that if all those assumptions hold, and these observations were correctly made, then the scientific finding in question holds.
For Existenz, this is pretty thin gruel. So what's the alternative? Well, religious folks nowadays like to make a similar argument against science cheerleaders and say that belief in science itself requires faith. Jaspers puts this argument in a new light. What's wrong with the argument as it's put forward now is that the world "faith" has been tainted, or perhaps misunderstood, as necessarily referring to the uncritical embrace of certain propositions. But that doesn't sound like what I just described, which is the provisional acceptance of what seem uncontroversial or unproblematic basic assumptions. If the data were to cast doubt on one of those (see our Kuhn episode re. potential problems with how this happens in practice), then presumably we would question it, as all material of science is supposed to be in principle falsifiable were contravening evidence to come along, unlike a religious person's bedrock beliefs in God, which remain untouched by any attempt to point out how awful and chaotic the world sometimes appears.
The kind of faith that Jaspers thinks is involved in Existenz is what he calls "philosophical faith," which is the kind of thing that Socrates died for. It would be unseemly, Jaspers says (in a different essay, "What Is Philosophical Faith?"), for someone like Galileo to die for a proposition that is subject to scientific proof, but philosophy is not about that. The kinds of conclusions that philosophers jump to are the result of efforts to reason at the limits of thought, and Jaspers distinguishes between "understanding," which is what science and ordinary thinking use to make sense of our experience, and "Reason," which has a broader scope, trying to encompass everything, even beyond possible experience. If you're familiar with Kant's epistemology (and if you're not, it's really essential that you go purchase our full episode on this), this will sound familiar: the "Critique of Pure Reason" is all about telling this runaway faculty of Reason that it shouldn't be so careless in, e.g. ascribing causality to the world-in-itself and so jumping to the metaphysical conclusion (following every Medieval theologian) that God must exist as first cause. But recall, also, that Kant immediately tried to get around his own restriction and had plenty to say about the role of "practical reason" in justifying our beliefs in morality, God, and teleology in the world. Jaspers simply wants to make Kant simpler and more consistent and so wants to legitimize in a way Reason's urges, not to make unfounded claims about reality itself and pretend that these are scientifically justified, but to engage in metaphysics, in philosophy, as a vital part of human life.
Following Kiekegaard (another old episode of ours you should really purchase to understand this), Jaspers thinks that philosophizing as an encounter with the transcendent is necessary for establishing a self; this is what Existenz is all about. Jaspers thinks that Nietzsche was urging the same think in instructing us to "become ourselves," that even though Nietzsche disavowed anything transcendent in the sense of otherworldly, he was urging the very same course of action, only using different imagery and conceptual apparatus.
And that's key for understanding what philosophy is about, and why Jaspers would want to call it "philosophical faith." When a scientist proves something via experiment, that scientific result is supposed to be replicable, and portable across different cultures. But philosophy goes further out on a limb, creating conceptual scaffolding (recall our Deleuze discussion in this context) that is much more dependent on shades of meaning that vary with culture and even between individuals. Per Wittgenstein, when we do philosophy we're talking about things that strictly speaking can't be talked about at all, so it's no wonder that it's so easy for us to misunderstand each other.
You may also recall that Hegel (whom Jaspers also found an endlessly fruitful read) emphasized that developing a self requires recognition by other people, and this came up as a question in our Kierkegaard discussion, as K. seemed to think that you didn't need another person, that your (admittedly one-sided) relationship with God could do the trick, that you could essentially see yourself through the eyes of God and that would be enough to give you a self. Jaspers is more in line with Buber here in saying that encountering transcendence can only really be done through another person. For Jaspers, you need deep immersion philosophical history to help you rediscover--from your own, inner source--ancient truths. And you need conversation with real, living people to really get a handle on these philosophical truths. They don't have to agree with you, but you need their perspective in order for this whole self-development-through-philosophy to be real and substantial.
So philosophical faith involves speech, and so philosophy does involve actual propositions, whose justification you can reflect on; it's not purely a matter of religious experience of the inexpressible. However, its subject matter is the transcendent, the inexpressible, so this kind of speech is very tricky. Certainly if you write it down and teach it by rote to people, and especially if you then demand that they believe it and that your formulation is the only route to transcendence, then something is going very, very wrong, and so Jaspers is very critical of established religions. Though the kinds of propositions philosophy is concerned with are of necessity not the kind of thing that science can provide full justification for, that doesn't get us off the hook in being as rigorous as the subject matter allows in believing only sensible stuff and in constantly questioning and re-questioning our conclusions. A scientific education (which should involve actually doing some science) is essential, Jaspers thinks, for really giving you a respect for rigorous inquiry. Philosophy is not against science in this respect, but merely stretches beyond science's scope; it should use science as a tool, but it also needs religion to inspire its primary subject matter.
It's characteristic of existentialism in I think all of its varieties that Man's destiny is to walk a razor's edge. For Camus, we're always tempted to settle on some false comfort and not properly face up to the fact of the absurd. For Sartre, it was about forgetting our own freedom and settling into bad faith. Heidegger had his own warnings about getting sucked into practical activity and the point of view of the They and losing sight of the question of Being. For Jaspers, the temptation is two-fold: if we let a philosophical conclusion solidify into a doctrine and pretend that this is certain and universalizable and take comfort in it, we're violating the dictates of our intellectual conscience, but on the other side if we stop leaping for the transcendent, give up the search for Truth with a capital "T," then we're also forfeiting our essential humanity. So keep the faith, but don't codify it!
If you want to purchase the reading, I recommend Walter Kaufmann's anthology Existentialism from Dostoevsky to Sartre, which also contains some really good excerpts from Jaspsers's Reason and Existenz, a lecture series from 1955 that more explicitly defines terms like "Existenz," "Reason," and "the Encompassing."
I also refer at the beginning of the podcast to The Heidegger-Jaspers Correspondence (1920-1963), which would likely help to clarify the the relationship between the ideas of these two fathers of German existentialism, and which also contains the only (sort of) apology that Heidegger seems to have issued for joining the Nazi party.
I also read The Perennial Scope of Philosophy, whose first lecture is "What Is Philosophical Faith?" and which goes into a lot more detail about the relationship between philosophy and religion.
I'm told that there's another lecture series that might prove to be a more comprehensive (in terms of the topics discussed above) while more readable than the material in the Kaufmann, Philosophy of Existence. It seems like he kept coming back to these same topics, trying to help people appreciate his approach.