We are alone. Each and every one of us is alone in the end. The very thing that makes us aware and not just alive, the spark of individuality, is at the same time a bleak wall or a yawning gulf; a cage to imprison us.
There are times when we can forget. Perhaps for a while the barrier will be breached when with eloquence we share what it is that makes us us. Words may serve as ladders to scale another's fastness, or silence as a space in which two can draw together.
This is an illusion.
How can it be anything but? I am because if I'm not I needn't worry about reality. The rest of you have no such guarantee. I may think I know you, but what do I know? If my model of a person is based, as it must be, on myself, can I ever know another?
And even if I'm reasonably certain you exist, how can we communicate meaningfully? I can't even be sure I mean the same thing you mean by 'apple', let alone something as complicated as 'red'. And as for 'mercy' or 'love' - not a prayer. We can build fragile towers of concepts in an attempt to bridge the gap, but in the end we are alone.