Yes you can. I obviously don't mean that you can decide not to be fat and the weight will magically disappear.
Every time you put food in your mouth, you're making a conscious choice, you're not "deciding to be fat" per se, but you are fully aware that too many rain drops causes a flood and that it will take a similar time of less rain drops for the flood to dry up.
If you commit eating less calories, you will lose weight, that's not an opinion, it's a fact.