Put one finger on a table top, and put your other arm under the table. Try to touch your first (on top of the table) finger with the finger of your other arm. Do it in such a way so you can’t really see your other arm.

Those fingrs will get pretty close to being tip to tip.

One part of that ability is knowing exactly where each of your arms/hands/fingers are because of neruons that are keeping track of your movements and sending feedback to your brain, but a lot of that is going on in your brain and not your limbs.

It’s like when we are at the cabin and my mother in law is knwon to be driving up as well, and my wife calls her.

“So where are you?”

“Rogers.”

Then later MIL calls “Did you want me to stop at the Best Buy and get that thing?”

“NO, that’s OK”

Then 15 minutes later I say “So, where are they?” and my wife says “Just north of Baxter, I’d guess”

Most of that calculation is in my wife’s brain; The information from “the field” (Mother in law, the arm, whatever) is vague. The Best Buy is in Baxter, so that call was made on the approach to Baxter. There are other best buys, but given the aount of time since “We’re in Rogers” and “Do you want me to stop at Best Buy” one guesses that it was Baxter.

In other words, proprioception is a complicated and subtle information system where much of the “inference” is happening inside the brain. The limb might be there or not be there, the inferences are still going to happen with only a little, or even no, input. Eventually phantom limb sensation wears away for most people.