How to tell PI you're quitting

Guys, I plan to talk to my PI about quiting the PhD in next week. What's the best way to convey that to him? My reason is that I can't seem to have ability or talent to do it. As it's been already 1.5 years and I don't have a specific plan or a problem to solve to. Have multiple problems and can't seem to find a path for a specific one. My PhD is for 4 years. I think I can't do it but my PI will ask me no, you can do it and will try to convince me to stay.