You are essentially correct in that purpose can come from anyone/anywhere. But the issue is not whether you can have a purpose (you can certainly give yourself one). Hitler had a purpose; it was to dominate the world and eradicate Judaism. He gave himself that purpose. Now, we all very quickly (and rightly) identify a problem with his purpose. But your statement makes this identification impossible. As you say "if you, God, or anyone else, want to do something with your life, then your life has purpose. Nothing else is relevant."
Nothing else is relevant...really?
Hitler's purpose for his life is just as valid as Mother Theresa's, or Ghandi's or someone else who lives a selfless life? If that is what your worldview really teaches, then it is inadequate because it fails to describe the world as it really is. Something else IS relevant; it’s the fact that we all know Hitler’s purpose was wrong. If your worldview can’t account for that, if it holds that the distinction isn’t even relevant, then it is missing a major piece of life’s puzzle...and you should re-evaluate it.
i think he meant that the origins of that life is irrelevant in the context of it having a purpose.
of course the effects of your purpose upon other conscious beings is relevant.
i hope he meant what i think he means.
This thread confuses the internal sense of "purpose" (I have meaning; I am not arbitrary") with moral codes ("This action is good; this action is bad"). The trigger statement "If man arose by chance, life would have no purpose or meaning" addressed purpose, not moral codes. And, indeed, purpose is internally generated and needs no external cosmic source. The sense of purpose is tightly linked to the sense of Self - the feeling that there is an "I" inside my head, the metaphorical centralized captain of the ship, the soul. (By the way, there is a specific area of the brain responsible for this feeling of centralized soul, and the feeling can be distorted or completely wiped out by damage to this part of the brain.)
As far as morality goes: of course social actions have social effects on others, and certain moral standards are indeed universal among human cultures. Believers would claim that universal moral codes (the Golden Rule, incest taboos, etc.) arise from God; but anthropology, neurology, neuropsychology, and animal studies say otherwise.
Humans are not blank slates that can develop any form of consciousness, perspective, awareness, and so forth. Human psychology and consciousness involves some hard-wiring. Many of the shared features of moral systems across cultures and societies have their roots in brain development, brain structure, and brain functioning. In other words: If we had evolved from non-social, asexual predators with extremely high reproduction rates, our brains would be different and our universal moral codes would be different.
One basic example is the Golden Rule, or the social value of empathy. Empathy arises directly from the ability to recognize that there is a Person inside another person - that when another person cries out in pain, their internal experience is similar to our own when we feel pain. This recognition that Other is like Self develops at around one year old in humans, when infants begin to cognitively differentiate themselves as individuals in a world of individuals. This is a predictable process that is directly related to the development of certain regions of the brain. In some people, this natural development is atrophied - usually through severe abuse and/or neurological damage.
So: Of course certain "purposes" are maladaptive, or immoral. But this doesn't mean purpose is handed down by a deity. It means that humans are animals, not angelic blank slates.
*** I'm NOT saying that morality/purpose is determined by brain structure and function; I'm saying it's influenced, and somewhat constrained, by those factors. *** There is a rich literature addressing the links between neurology and social behavior which I urge readers to explore. A fun one is "The Man Who Mistook His Wife for a Hat" ...
After 35 years as an agnostic, I was on a jury and on a lunch break with two other men. One said he was an Episcopal and the other said he was a Catholic priest. I said I had once been a Catholic and the priest said my life was absurd.
Having days earlier seen "absurd" defined as meaningless, I replied that giving my life meaning was my responsibility. He so suddenly went silent and left the room that I decided my remark had frightened him. Remembering the twelve years of mental bullying I had survived in Catholic schools, I started telling myself, "Don't tell me revenge isn't sweet!" What fun it was.