Moderator note: This thread was originally split from two unrelated threads titled "arfa's musings on elastic surfaces" and "Time and information". A conversation about whether mass is a number came up in both of those threads. Here is is consolidated in one place. After you, James. What I'm actually interested in is why mass, distance and time are convenient abstractions when you compose them in a certain way and call it energy. Of course, mass is something you can put in a bottle. Or maybe that depends on how you abstract what a bottle is, whether you can say a bottle is real. Maybe it's down to whether anything is real, or whether physics is about anything real or whether real is an appropriate concept. Time is an abstraction, right? Distance is real though, or maybe just really an abstraction which is useful. Maybe physics is just one big abstraction; the moon abstractly orbits the earth. The sun abstractly outputs light and heat. Yeah . . . Oh thankyou so much, James. I get to discuss the important ideas with someone as important as you. I feel so privileged. Oops, I'm wrong again. So when is feeding time?