The only reality for human beings is what they feel and experience. Take the case of someone who has hallucinations: they scream in terror because they feel that they are being chased by monsters.
Visibly, no one is physically attacking them, but they feel persecuted and are suffering, and when someone is suffering, try telling them that it is all an illusion!
Then there are those who experience ecstasy and illumination in spite of appalling conditions. There again, how can you convince them that they are deluding themselves? The sufferings and joys people experience are perhaps the only things they never doubt.
We can doubt what we see, what we hear and what we touch, but we can never doubt the reality of what we feel and experience.