I know there is a word to describe the act of doing exactly what one wants to do, even though one knows one oughtn't.

For example: I know I shouldn't eat this entire tray of brownies but I will anyway, because I want to.

This concept can be expanded to define a philosophy that asserts that people will always do exactly what they want, because if they didn't want to do it, they wouldn't.

Perhaps it ends in 'esis- or 'gesis'? For some reason I'm wanting to say "Achresis" but I'm pretty sure that word just doesn't exist.

I've been up and down the internet, my dictionary, and my bookshelves (because I know I encountered it in a book sometime in the past year or so) but I'm still at a loss. Any help would be greatly appreciated.

Thank You,
val