Seeing vs Thinking a Painting

I believed for years that I was painting realistically. After reading the tips on painting realism, I'm not so sure. Paint what you see and not what you think you see? How do I know the difference? -- Mark Simonhoff

For me the "Eureka!" moment was in a self-portraiture class where the tutor said: "It's a good eye, but is it your eye." And it wasn't.

With realism, it's not enough that what's on your canvas looks real. An apple can't merely look like an apple, a generic apple, a perfect apple. It must look like the specific apple in front of you and only that individual apple. That's the difference between painting what you see and what you think you see.