There's an interesting essay on how Thais see personality traits, self and soul with the respect to Buddhism at thaibuddhism.net. For example:

Sandan (Pali, santana; continuity of subliminal consciousness) — This term is mentioned first because it can be related to both nature (genetics) and nurture (upbringing). According to its Pali roots, this concept tends to mean the ever-moving, ever-flowing panorama or continuity of successive mental states; in more common Thai usage it is held to be the intrinsic or inherent qualities of character, inborn traits, or innate character (even lineage, though rare). There is a Thai aphorism that says, sandon khut dai, tae sandan khut mai dai (a sandbar can be dug away, but innate characteristics cannot be dug out).

Interesting article, thanks camerata. I know someone who was an inveterate gambler and when he quit gambling everyone (and I mean everyone) was saying that he would eventually go back, that he couldn't change. At the time I put it down to small-town thinking and unwillingness to allow someone to change. Now I see that it perhaps has other roots as well.

Thais talk about sandan a lot, as the unchanging part of a person's character, almost always in the negative sense.