When people ask for a meaning of life, they are asking for life's purpose, justification, or goal -- not, a "meaning" in the sense in which words have meaning. This is why responses such as 'life can't have a meaning, it's not a word' or 'look it up in a dictionary' are fallacious or spurious. The definition of life is an interesting issue in its own right, however, especially as relating to artificial and extraterrestrial life.
We can also separate this question into two different questions; one about the objective purpose of life, and the other about subjective purpose of life. The subjective purpose of life varies of course from person, and need not be considered any further.
Many deny that an objective purpose of anything is possible. Purposes, they argue, are purely subjective. Others claim that life has an objective purpose, though they differ as to what this purpose is, or where it comes from.
Topics that one might contemplate, related to the meaning of life, include:
Over the millennia, philosophers have had much to say about this question--though philosophers do not fixate on it as much as popular conceptions might lead one to believe. Theories of value--of which there are very many indeed--are not necessarily, but can sometimes be construed as theories of the meaning of life. Great philosophers such as Socrates, Plato, Aristotle, Descartes, Spinoza, and many others had clear views about what sort of life was best (and hence most meaningful). The existentialists addressed themselves to the question head-on. More recently, Robert Nozick discussed the question at great length in his Philosophical Explanations.
Using a general line of thinking exemplified by Wittgenstein and the logical positivists, it could be said that, expressed in language, the question is meaningless. This is because 'meaning of X' is a term in life usually conveying something regarding 'the consequences of X', or 'significance of X', or 'that which should be noted regarding X', etc..
Things in my life can therefore be said to have meaning (for me, for other people): my life can even be said to have meaning (legacy, achievements, family etc).
But to say that life itself as a whole has meaning is a misuse of language, since any note or significance or consequence would be outwith life and therefore highly dubious in status. The Wittgensteinian line would say therefore that language cannot provide a meaningful answer unless it refers to a realm outwith the realm of life, but this is not usually given.
Religion itself, it is often suggested, is a response to humanity's search for meaning or purpose. Indeed, the realm outside life itself referred to in the previous passage could be interpreted as the religious or spiritual realm. Most people who accept God in their lives would agree that it is God "in Whom we live and move and have our being". The notion here is that we do or ought to seek a higher purpose that will give our lives meaning.
However, this does not help the lay person in dealing with the question which is asked in a lay context, "What is the Meaning of Life?". It is not a generally acceptable answer to say, although perhaps a required answer from the religious point of view, "Believe, and you will understand".
Nevertheless, for a sizeable portion of humanity, that is the answer to the question: for the rest, it is reasonable to say (from a Wittgensteinian point of view) that it is a meaningless question.
Science is sometimes criticized for not providing an answer to "the meaning for life", but it does not attempt to do so. Science addresses questions of "what" and "how", but does not attempt to answer "why".
A Mathematical view can be found in the Incompleteness Theorem, which basically says that there are some Mathematical facts that are true yet unprovable, unless you adopt a view of the universe that is so simple that it does not include arithmetic.
A Computer Science view can be found in the Halting Problem, which says that it impossible to know if an arbitrary program will eventually finish running, unless you cripple the allowable programs in some fashion. (For example, you can require that each program be allowed only a finite amount of memory. In this case, it must eventually repeat a state, so it will eventually halt or go into a recognizable loop.)
Some people say that the Heisenberg Uncertainty Principle also follows this theme, that you can't possibly know everything about a particle.
The number 42 as an answer to the question of the meaning of life is a reference to a joke in Douglas Adams's book The Hitch-Hiker's Guide to the Galaxy. An advanced race of pandimensional beings builds a gigantic computer called "Deep Thought" to find the Answer to "Life! The Universe! Everything!". Seven million years later, the computer gave the answer: "42". After the answer was given, the pandimensional beings realized that they did not know the question and an even larger computer (the earth) was built to find it, however the earth is destroyed moments before the final readout.
Another notable non-serious view is Monty Python's The Meaning of Life: "The meaning of life is to live."
Jane Roberts in the Seth books gives Seth's distinctive views on the meaning or purpose of life and Seth's view can be paraphrased as "the purpose of life is achieved by being not by doing". More specifically in the books that Seth once described as his masterwork (see references below) Seth introduced the concept of value fulfilment. He said that the concept was difficult to verbalise but meant something like achievement of self-expression. In his works Seth argued that all things are conscious (trees, animals, the environment) and that each conscious entity seeks to more fully realise it's potential i.e. it seeks value fulfilment...the exploration and growth of its own way of being.
Agent Smith in Matrix Revolutions states: "The purpose of all life is to end."
What does it mean to ask what the meaning of life is?
How philosophers have addressed the question
Religious views on the meaning of life
Science views on the meaning of life
Other views on the meaning of life
References
External Links