This is an excellent question, and one I’ve considered often in the last decade-plus of working with such folks.
First, it depends upon the stage of dementia: mild, moderate, or severe.
In mild dementia, it seems to be like being a functional alcoholic’s day, as far as cognition goes. You’re able to do what you need to do, but some little things get missed, such as your T-shirt is on backward, but you don’t notice, or you can’t find the sugar bowl, so you start taking apart cupboards and end up going without coffee and the kitchen is a mess. Later, you swear you did not do that. You have no memory of doing it, and the more another person argues that you did indeed make that mess, the angrier you get. You did not. He or she is lying.