Since I didn’t have the time to enter into a long discussion over the metaphysical ramifications of what he was asking (after all, in the grand scheme of things, we all are, at least on some level. In some traditions, it is even desirable to recognize your intangibility as it is a reminder of the impermanence of the self...), I opted for the simplest answer that came to me. "Nope. I can see you just fine."
"Well ok then," he replied. "Make sure you tell everyone else that."
So anyway, there's a guy at the end of Boylston who, while not exactly pleasant to look upon, is decidedly not invisible (in the strictest sense.) Even if you don't hand him your spare change or have the time to stop and chat, give a nod in his direction to let him know that you can at least see him.