This ancient mysterious question, the topic of eminent philosophical debates, religions and secret ritual initiations, could perhaps be answered, at least in part, by looking to the contributions of Evolutionism and modern Psychology. Evolutionism describes, as objectively as possible, the processes, tendencies and laws of the developing universe. Psychology describes the deepest needs and aspirations of man, and how they can be durably fulfilled.
During the 20th century both these sources of fundamental knowledge, objective and subjective, were dramatically elaborated, and appeared to yield the same conclusions. As is argued elsewhere (Integration), the fact to use several complementary sources of knowledge adds evidence to the plausibility of this description of the Sense of Life and Existence.
Objective, evolutionary evidence
When we look at the fundamental tendencies of the Evolution of the Universe, we observe that a progressive increase of complexity and, impressively obvious in the higher levels, consciousness. In fact, both are interrelated. From a certain point, complexification is only possible if some kind of central organization is introduced, and consciousness appears to be the most elaborated form of such an organization. On the other hand, higher forms of consciousness require a rather highly developed form of material (i.e. biological) complexity.
Both tendencies illustrate an impressive, never ending strive to build up ever more complex systems, and although competition, mutual aggresiveness and destruction occur in the lower stages of this development, the eventual tendency appears to be cooperation, complementarity, synergy and integration.
In fact, this integrative synergy and structuration is the way nature tries to "satisfy" and "appease" the instabilities and incompletenesses existing at each level of universal development: e.g. molecule formation is a way to appease the need for a "completed" outer electron shell ("octet structure"). This mechanism is repeated at each level of complexification.
Observed from this external, objective point of view, the essential characteristic of existence in the universe seems to be: to create and to develop ever more complex forms of interaction and systemization, at the same time enabling and employing even more developed forms of central organization (becoming conscious at the highest levels of evolution).
Subjective, psychological evidence
Freud, Jung and Maslow and a host of other creative psychologists described, from the early years of the 20th century on, the fundamental psychological drive, often unconscious, to fulfill his needs and desires, partly innate and partly developed during the early stages of personal development.
The confirmation by neurobiochemical research, that the brain functions along a constant need of reward, describing in detail the pathways and neurotransmitters realizing this function, freed these theories form the mists of speculation.
As it appears, a congruous and elaborated fulfillment of needs converges into a constructive approach of life and reality. The best and most pleasing way to enjoy all kinds of happiness, is to engage in significant and creative contributions to scientific, technological, artistic, social and communicative realizations. To best way to feel happiness is to create happiness in all its possible forms. The fertility of creativity and the intensity of the feeling of happiness seem to increase with our consciousness about the processes we engender.
The Sense of Life and Existence
In accordance with these two sources of knowledge, the Sense of Life and existence could be formulated as stated on our Home Page:
Existence is the progressive development and integration of ever more complex and conscious structures from strings and quarks up to a socialized planet (and universe). Human life is the opportunity for a conscious participation into this universal integrative evolution. The most central process in this universal and psychological realizations is the phenomenon of integration. This process, that occurs spontaneously in all forms of evolutionary progress, and that also can be consciously performed by intelligence, consists in a constructive combination of dynamic elements, factual or conceptual, often hidden in a non-integrative and even confluctuous shape.
Most of religious, philosophical and even political theories can be interpreted as an intuitive approach to this formulation. But the lack of universal applicability of most of these theories often allowed distorted and sometimes dangerous conclusions, as illustrated by every kind of indifferent, or even aggressive and destructive behaviour perpetrated with the endorsement of moral, religious and/or political authorities.
The fundamental Law of Existence
We could put one more step inwards, and try to define to most fundamental Law of Existence. Although Integration is a very important process, it is perhaps only the concrete way by which a deeper law is applied. To integrate is a way to participate into the existence of other systems, things and people, but what is this drive which pushes us towards active participation?
I propose the law: To exist = to educe existence
or to generate existence.
Description. This law, that I formulated first in 1966 , states that every form of existence (every system) somehow serves to provoke, sustain, repair, perfect or render useful other systems. It also suggests that each system obligatorily fulfills such a function, and that nothing exists without that effect. Moreover, each system in the universe can be seen als belonging to one (or several) chains of existence, containing two to innumerable participating systems.
Of course, doing so is not the final motivation of existing systems: they are only interested in their own needs: atoms tend to complete their outer electron layer ("striving" to an octet structure), animals try to eat, to copulate, and to prevent being eaten, and humans are driven by a number of inborn instincts, and to fulfill their physiological, sexual and psychological needs. This combination of often conflictuous motivations not automatically leads to constructive particpation into general existence. Often systems are destroyed (although being eaten is also useful function!), and sometimes systems, especially humans, act dangerously and even commit suicide. So, this law is not a guarantee for immediate and short term constructive effect. But when a number systems interact a significant period, the most durable mode of interaction will be the constructive, otherwise the evolutionary probability that they will disappear is more likely: existence can be destructive, but omn the long run only constructive, integrating participation into the existence of another people and things, yet into the universe as a whole, is the only durable interaction mode.
In humans, and to a certain degree in higher mammals, this fundamental process occurs, or at least could occur, on a conscious way: we can consciously project and decide to participate on an integrative, constructive way into the existence of other systems and people, and feel happy --another, intensive form of consciousness-- we do/did so. The fundamental law, for conscious systems, should read:
To consciously exist = to consciously educe existence and feel happy by doing so
Because we humans are not only driven by instincts, but also by subjective interpretations about our own functioning and importance (called phantasms by Freud), ons could state that, at this highest level of functioning in the Universe, i.e. the Noosphere, the phantasm to actively participate and to be of importance in the existence of others is perhaps the most central, which is confirmed by psychoanalysis.
Arguments and references. There is perhaps no better definition of love than "to consciously educe (happy) existence in somebody else, and feel happy by doing so". Teilhard said somewhere (and Brian C. probably will find the reference :-)) "Love is a local form of a universal process".
Even Einstein, in his fizzing mind, concluded that E = m.c2
or: To exist (mass) is to radiate effect (energy), and in extreme, nuclear conditions all mass is transformed into energy, and vice versa. This analogy is more than striking. Both statements perhaps point to the same fundamental reality
If we look to animal bodies, e.g. human body, we can roughly discern five subsystems, called tissues: muscles, connective tissue (including skin and bones), blood, gland tissue and the nervous system. The only sense of each tissue is to sustain the other four tissues. The significance of each tissue can only be evaluated in what it does and means for the other tissues. Biologists, and especially ecologists, design thousands of cycles describing the mutual interactions and transitions. Nature seems to be an astronomical number of mutually equilibrating although vulnerable cycles. The more complex the natural processes we study, the more complex the existential cycles (to exist = to educe existence) we can observe. The same applies for sociological processes.
An argument ex absurdo could be that the Universe had no choice: should another law have been fundamental, e.g. "to exist is to use the existence of other systems to fuel its own", universe should not have been long lasting. Or "to exist is not to educe existence" -- in that case we shouldn't be here to reflect about that hypothesis.
The most impressive application of this law is, of course, universe itself. We see that, from the very simplest subatomic particles to socializing humanity on, nature never rests in trying to develop even more complex chains or cycles of existence. Interaction and centralization gradually become more expanding, more global. Layer after layer one degree of complexity is added: the products of the former layer become building blocks of the next layer. By the same process, interdependence and vulnerability increase, and the margin of conditions, within which the evolution can occur, even more narrow. This interdependence and vulnerability only can be protected by even more complicated existential cycles.
2. The God Hypothesis and its uneasy consequences
The most intriguing applications of this fundamental law can perhaps be found in some problems concerning God. These considerations are, of course, highly speculative, as speculative as the existence of God itself, but it is comforting that these intriguing hypotheses seem to comply with this fundamental law, and even find (a shy beginning of) an explanation in it.
1. Pain and imperfection. If we consider the God hypothesis, the first problem with God is: why did He create a Universe at all, and by doing so, why did he create such a creeping and painful universe? He is supposed to be a loving God and an Almighty, but His creatrion --if it really does come from him-- rather seems to be an act of incompetence and sadism. If our Fundamentel Law is right, God had no other choice to start up a Universe. In fact, this could be His only occupation, His only "Sense of Existence". One can, of course, use Love terms for it: God is Love .
Suppose he had immediately created a perfect universe, to avoid all that "useless" pain and spill of energy. This looks much more attractive, much more humane, but was it feasable? To create an auto-poietic (self-making) universe seems to be much more in compliance with the fundamental law "To exist is to educe existence". To enhance existence, to perfect universe is our only sense of existence! When a creator should have put us right into a perfect universe, our existence should have been useless, senseless. And, as discussed in the "Pro-creation Hypothesis" at the end of "Beyond the Evolving Universe", the perfected Universe probably immediately will start to set up a new universe, most probably along the same auto-poietic lines.
Of course, such a statement seems merciless and highly arrogant with regard to the indescribable sufferings by barbaries and senseless accidents, tragedies and illnesses. This important question, including the problem of death and suffering (Teilhard wrote about it), the intuition of life after death, still remains greatly unanswered, although this existential principle sheds some light on it. Some pages (yet to come) and links on this website try to discuss some of them.
2. Trinity. Another problem concerning God is the mysterious Trinity: one God in three persons. Being three in one is, of course, a tricky way to resolve the problem of solitude. But I think deeper considerations are indicated. As existing is educing existence, the very nature of God also ought to be a "Chain of Existence". When we consider the Third Person, the Spirit, as the interaction between Father and Son, we have here the most simple form of a chain of existence: two entities. I'm convinced Teilhard thought along the same lines, calling the created, evolving universe a Christogenesis. In other words, along Teilhard the Trinity is still in a stage of formation, although the limiting frame of time probably will not apply to God: from His point of view, Trinity --and Universe-- are already completed. [This topic is more extensively discussed in a page on the Christian Inspiration of Teilhard.]
Another fascinating application of the law "To exist is to educe existence by integration" is the optimistic approach of evolution. In modern culture it seems wise to profess a certain pessimism. Optimism most evidently is a sign of naive credulousness. Suffice it to look at all the troubles and miseries in the world, most of them provoked by man himself, to conclude to the very bad nature of man, and the tragic issue of existence itself. Teilhard got some hard problems with his optimistic approach that, on top of his other statements suspected for heresy, seemed to deny original sin and hence the whole usefulness of salvation and church. This seems incredible for someone who wrote some of his most enthralling essays in Flanders Fields, and his Opus Magnum, The Phenomenon of Man, at the eve of World War II, in China, already under Japanese attack. He added a last chapter treating sin and evil, but this couldn't deceive his censor.
Pessimists use to label their approach as realistic, pointing at the perpetually returning tragedies, perhaps gradually even more atrocious. Optimism is considered as highly irrealistic, a claim without proof. Nevertheless, the opposite seems more plausible. In fact, optimists just believe that things eventually will evolve the way they always evolved. The thesis of a pessimist is: "in the former 8 levels of universal evolution, constructive outcomes were realized. But here and now, in the 9th level, the whole construction will collapse: integration will not be reached". On the other hand, the optimist is more traditional, more scientific. He just states: "Already 8 times, in much more difficult conditions than we have here and now, universe succeeded to make succesful integrations at each level. Now, the 9th time, it will occur probably the same way". It is important to see that the optimist has not to justify his position: he simply thinks things will remain as they always were. It is the pessimist who advances a proposition, completely opposite to what happened in the past. Suddenly, this universal law would be reversed. So, it's up to him present good arguments and proofs, not to the optimist! Of course, there is other law of nature that states that natural systems always produce more trials than the expected result: there are more flowers than apples, and more apples than new, young aplpletrees. Although the balance between trials / results tends to 1 with more evolved systems (fishes have thousands of little fishes, modern man roughly two children per couple, with few or no miscarriages), the certainty to hit the final outcome (the Omega Point) probably not yet equals 1. So, we could assume that, on a few thousand inhabited planets in the universe some probably will fail at the last stages of the evolution, and the Earth could be one of those unhappy planets. But even if the Earth fails, the global outcome most likeky will be positive.
A related phenomenon is the constructive "attitude" in life. In the aftermath of the Hippie movement, perhaps the first sign of the new Renaissance, during some decades numerous books about positive thinking were published, the most renown being V.N. Peale's The Force of Positive Thinking. . The essence is that, while the possibilities of our personality and the situation we're living in, are probable richer than our defensive, neurotic way of thinking suggests, our chances to discover them are bigger if we suppose they are real. Moreover, new opportunities most probably will arise as a direct consequence of a positive attitude. Although this hype passed over with the desillusions of the eighties, the kernel of this idea remained, as well in psychotherapy as in commercial circles. Also this personal form of optimism, rather inspiring to constructive action, creativity and self-deployment is clearly in line with the "attitude" the constantly evolving universe always featured, leading to the creation and realization of systems, highly unlikely in the prevailing circumstances. Positive and constructive thinking is not just one variant of several possible appropriate mental attitudes, it is the fundamental law of evolving nature.
 Roose, Kris, Hiperfizika, zin en toekomst van bestaan en heelal, ("Hyperphysics, Sense and Future of Existence and Universe"). Ms., 1966.
 Bible, 1 Joh 4 and 18
 I myself published one in Dutch: Roose K. & Van Brandt B.: Het Geheim van het Geluk ("The Secret of Happiness"), Kluwer, Antwerpen, 1985.