@hue-man,
I agree with Kennethamy, but I don't know if this view was 'Cartesian'. Decartes was Catholic, and believed in the existence of the soul; and presumably 'the soul' was both essential, and precedes 'existence'. The 'existence precedes essence' argument is based on the denial of the soul (or anything covertly or overtly religious) - it is a reversal of the traditional idea that the 'essence' of something or someone is more fundamental than their existence.
It would seem to me that the 'nature vs nurture' argument should have some bearing on this view. Studies of twins seperated at birth show that in many uncanny respects, their lives, attitudes, actions, tastes and so on are very similar, even if they have lived apart of all of their lives. So this would seem to me to support the 'essence' view.
But then you can interpret 'essence' to refer to 'collective memory', cultural and genetic inheritance, and the like, rather than being something immutable or eternally fixed. In this sense, the 'essential' aspects of a personality could still be seen to be something that one is born with. So again, 'essence precedes existence', even if 'essence' is not something immutable or eternal.
---------- Post added 01-01-2010 at 05:36 PM ----------
Incidentally in regards to Pirsig, it has been about 3 decades since I read 'Zen and...' and can't recall much of it.