This is an interesting discussion and one I find very valuable in my continuing process of working on "my definite purpose in life".
Here are a few things that might be helpful to some here, perhaps to the discussion and certainly have been helpful to me:
1) Another version of "definite life purpose" is sometimes known as a "mission statement".
2) Whatever you call it, and whatever approach one uses,
IT IS NOT WRITTEN IN STONE! I.e. you can change it whenever and however and however often you want! Buckminster Fuller taught that goals create what I think he called "vector energy", this is the energy that keeps you moving in a positive direction. He showed it as a wheel and the goal is point on the wheel and moving toward the goal moves the wheel. But either as you approach or once you get there, you can change your mind and change the goal. And that sets up a new vector energy, keeping the wheel moving and moving you in a positive and fulfilling direction
3). Intutition, imagination, vision all are very important to me in terms of working on definite purpose in life. However, there are rational approaches or at least approaches that use more of the left brain or analytical mind to come up with a definite purpose.
For example, Stephen Covey's 2nd Habit of his 7 HABITS OF HIGHLY EFFECTIVE PEOPLE is "Begin with the End in Mind". And when he says "End" he
really means end! As in, the end of your life! And so, the exercise he has you go through is to write down what you would want those people who are (or will be) important to you at the end of your life: spouse, children, grandchildren, friends, clients, co-workers, colleagues, students, whomever, to say about you after you die. After doing this, try to kind of write a eulogy for yourself, in other words, what kind of person would you have to have been to have those people say that about you, whatever you would want them to say "He was a very loving man", "She was loyal woman", "He was a hard worker" "She was a healing presence" ... whatever. From there, you try to come up with a "mission statement", simple statement that sums up what your mission or purpose in life would have to be for you be that kind of person.
This is a very effective (one might even say "highly effective"

) way to focus on life's purpose.
And that brings me to...
4) While I certainly salute those who say they neither want nor need to come up with a definite life purpose and if that works for them in being happy, successful or whatever, my hat is off to them, I believe that for me and for many of us we really need to focus a little more in order to be successful and/or happy. I really believe Napoleon Hill when he lists this as one the essential elements to riches, financial success, relationship success, peace of mind, whatever, not only for him and many of the giants of success that he studied for 20 years (!) to come up with these principles, but for me and those like me who really want to make significant change in this direction of success.
I keep seeing, reading, and hearing this idea in so much of what I am studying these days and things coming into my life. It is a guiding principle in the PhotoReading Whole Mind System, as well as so many of the Paraliminals. It is one of the main principles in the movie and book THE SECRET (so much of which owes so much to Napoleon Hill, that they should give him more of a credit if not royalties. On the other hand, this idea is something that is age-old wisdom and Napoleon Hill "discovered" or uncovered it in away that makes it easy to teach and understand, but it proceeds him by millennia.
For me, that is too much coincidence to be "mere accident". I feel it is important, at least for me. I must admit that for me it is also difficult. I have trouble with this element in some of the Paraliminals and even when I am ready to do the Prepare step in PhotoReading and also when I work on affirmations and visualizations. But just because it is hard (at least at first) does not mean, for me, that therefore I should avoid it. Rather, it shows me more how important it is.
I think that in so many ways, we really can get so much more of what we want, not only to help ourselves, but to help others and even to do it at the same time! It seems like a paradox, but I think when one "stumbles" onto "The Secret" or whatever one wants to call this true principle, this part of it, being focused on a definite purpose is the real engine in making "miracles" or "magic" happen in one's life. I think it is Napoleon Hill who says, that that is one of the main reasons why people fail to be rich or whatever measure of success they are looking for: i.e. the
lack of a definite purpose. And conversely, so many of the people he studied: Helen Keller, Henry Ford, Thomas Edison, Woodrow Wilson, Wilbur Wright, Andrew Carnegie (his main mentor and the man who started him on his path and his "definite purpose" to teach a "science of success"), Alexander Graham Bell and so many other giants, all had a definite purpose and Napoleon believed this was a very important part of their success.
And again, even in some of those examples, this does not mean that once you come up with one, it can't be changed. On the contrary, one might stumble onto something even better and your purpose might change for the better (or better for you). Hill gives the example of A.G. Bell trying to invent a hearing aid for his wife. He failed. But in so doing he stumbled on that which became the telephone.
Focus on definite purpose, mission, goal whatever you want to call it or come up with it, is a driving engine to success and fulfilment. It could certainly be "My definite purpose is to have no definite purpose"! Or "I just want to discover what life reveals to me as I move through it". Or "My definite purpose is to be happy" Or spiritual, or a person who learns a lot or whatever. And it could also be "to be aimless". But the more one knows who the "I am" is, what they want in and from life (could be enlightenment or peace of mind) and has some at least general plan on how to accomplish that, the more one will be fulfilled or at least will fullfill some of what one wants to do.
All of that is in the category of for what it's worth and your milieage may vary.
It helps me to write it. I hope it helps somebody to read it.
All the best,
Steve