Give me your best argument against Solipsism

There's an old joke that Alan Watts used to say about solipsism, saying it was laughable, but he knew people who took it seriously. His one wish wwas to go to a an International Conference for Solipsism and see the atendes argue over who was the real one.

It seems to me that an enormous amount of western philosophy and Indian philosophy deals with questions related to the self-other or subject-object (or atman-brahman) distinction.

I was thinking about this when I wrote that post, because the Upanisads have that well known declaration "you are that/tat tvam asi." This is literally the exact opposite of what I said earlier.

I was also thinking of a passage from Mark Twain,

"Then, all tranquilly and soberly, he made the strange answer, “There is no other.”

A subtle influence blew upon my spirit from his, bringing with it a vague, dim, but blessed and hopeful feeling that the incredible words might be true—even must be true.

“Have you never suspected this, Theodor?”

“No. How could I? But if it can only be true—”

“It is true.”

A gust of thankfulness rose in my breast, but a doubt checked it before it could issue in words, and I said, “But—but—we have seen that future life—seen it in its actuality, and so—”

“It was a vision—it had no existence.”

I could hardly breathe for the great hope that was struggling in me. “A vision?—a vi—”

“Life itself is only a vision, a dream.”

It was electrical. By God! I had had that very thought a thousand times in my musings!

“Nothing exists; all is a dream. God—man—the world—the sun, the moon, the wilderness of stars—a dream, all a dream; they have no existence. Nothing exists save empty space—and you!”

“I!”

“And you are not you—you have no body, no blood, no bones, you are but a thought. I myself have no existence; I am but a dream—your dream, creature of your imagination. In a moment you will have realized this, then you will banish me from your visions and I shall dissolve into the nothingness out of which you made me...."

So I agree there is more to this matter, but I myself am quite convinced that the world exists and more or less for the reasons previously outlined.

I never was sure just what the Upanisads were getting at. It seemed more a mystical dissolution of the self than an assertion of self as God or self as All.