Given that, there are two, and only two possibilities:
Either
-my perception of reality is inherently opposed to objective reality
or
-the world, for some reason, is gaslighting me into thinking I ate a sandwich and the objective reality is that I ate a grapefruit.
See, my problem with this "philosophy" that objective reality does not exist is that it enables abusers. Have you ever seen the movie Gaslight? Its a classic. This poor woman lives with an abuser...someone who is committed to making her think that she is crazy. He contradicts everything she does and says, sets up evidence to objectively prove that she only imagined herself doing it, and keeps her under his thumb through those means. In the end of the movie, its revealed that none of the evidence is real and that she is sane. To combat against these types of people, its important to be sure of your own reality and only change if evidence is overwhelmingly pointed in the opposite direction. Even then, question these changes in belief heavily. Otherwise, you will believe just about anything anyone says.
So sure that regardless of what evidence you are presented, you'd be certain that you ate a grapefruit. Even if:
- I showed you videoproof that you were eating a sandwich for breakfast
- we had all of your family say they were having breakfast with you and saw you eating a sandwich
- a doctor came and said "I analyzed your stool and found no evidence of grapefruit"
- we had a message, cryptographically signed by a key you generated/controlled, that said "man this sandwich is delicious"
Even with any amount of evidence to the contrary, you'd still believe that you ate a grapefruit?
If not, then are you sure that it's fact that you ate a grapefruit, or it's just that all current evidence points to you eating a grapefruit?