From approximately age 35 to 56 I was intensely engaged by the subject matter of my work; and at such a depth that I was working creatively, with considerable originality.
I was perfectly aware of this transformation in my life, and indeed can date it to a specific moment, on a specific spring day - sitting in the garden! Halfway through a Biblical lifespan; I at last found my subject, and I switched-on.
What characterised this era is that I gave my best efforts to the matters I was considering; this amounted not just to reading-about them - but reading with a genuine, seeking attitude. And, most especially, thinking-about the current problems - thinking intensely for considerable stretches of time - from half an hour up to a few hours (which was all I could manage), and also returning to the problem day after day, month after month - sometimes over several years.
I had not really done this before. From schooldays, through being a student and my early years of academic work; I was pretty successful. But I had never given my best efforts to it; since I was always more interested by other things - and regarded myself as primarily an 'all rounder' or Renaissance man who had significant interests in literature, music and drama - as well as medicine and science; in journalism as well as academia. Always I was working 'part time', semi-engaged...
It took me until age 35 to develop my own knowledge and abilities sufficiently to do original work; and also to bring them to bear upon a series of subjects which genuinely motivated me - which triggered spontaneous inner motivation. These subjects included evolutionary biology, psychiatry and psychopharmacology, some epidemiological/ statistical issues to do with randomisation and averageing, complex systems theory, intelligence and creativity, and the metaphysical assumptions of biology.
The work I did during those 21 years included things that were qualitatively superior to anything done before: more personal, distinctive, original. The work was different from the mainstream - was not simply filling in gaps in existing work, nor extrapolating from existing work using standard methods. At times, I felt I got significantly deeper than the mainstream; and I made discoveries that were not simply variants of previous discoveries.
I suppose it goes without saying that the work I did during those 21 tears was less successful than the work I did before I became creative and original; but that didn't really matter to me because creative and original work is its own reward; and the reward comes immediately, while you are actually doing it.
I don't get any particular 'satisfaction' looking back; I am not terribly interested by the work I was doing a quarter century ago. Not that I regard it as wrong or poor; but simply that the satisfaction came at the time. Not many people were interested then, and even fewer are interested now - so it goes.
Although life is cumulative, and that which is good and true and beautiful is permanent and cumulative - satisfaction is not, I find, a thing that can be stored for very long. It seems we are made to be-living, to look forward - and I suppose that this will continue beyond the portals of death. Creat-ing (present tense) is always more satisfying than past creat-ion.