Science. For the most part, anyway. Some things aren't quite as simple. Some things can only be experienced through emotion.

[ Some of it, most of it, he finds genuinely breathtaking. Science can't possibly explain how these things make him feel sometimes. ]

Yes, exactly. One reality, one universe. Some are very similar, others are completely different. Some might even be practically identical, and there can be an infinite number of realities that we will never even know exist.

[ He cants his head. ] Well, you can tell me a little about your Earth, and I'll tell you about mine, and we can see if they match up?