You are not alone - even in your own consciousness

"There are thinkers who believe that a special
difficulty arises when one tries to understand how another person's thought life can affect one's own.  They say: my conscious world is enclosed within me; in the same way, any other conscious world is enclosed within itself. I cannot see into the world of consciousness of another person. How, then, do I know that he and I are both in the same world?"

Rudold Steiner presents this problem in the Appendix to Philosophy of Freedom. I think it's still an interesting question today, and still a vital question, if not in those exact terms. His answer is not an argument, but a description of the experience of encountering another human being in real time:

"The immediate percept [of the person's physical presence], extinguishing itself as sense appearance, is grasped by my thinking, and this is a process lying wholly within my consciousness and consisting in this, that the other person's thinking takes the place of mine. Through the self extinction of the sense appearance, the separation between the two spheres of consciousness is actually overcome."

That is, "I have really perceived another person's thinking."

If you will pay close attention the next time you encounter and engage with someone, you'll notice this for yourself. This accounts, too, for the difference, in quality of experience, between reading someone's email, for instance, and talking with them in person: the physical presence isn't there to begin with - all you have is their thinking taking place of your own.

Tell me if I'm wrong.