What is “the legend of the shield”?

On page 457 of her book Science and Health with Key to the Scriptures, Mary Baker Eddy states, “That we cannot scientifically both cure and cause disease is self-evident.” She follows that assertion with this illustration:

In the legend of the shield, which led to a quarrel between two knights because each of them could see but one face of it, both sides were beautiful according to their degree; but to mental malpractice, prolific of evil, there is no good aspect, either silvern or golden.¹

While we do not know Eddy’s source for “the legend of the shield,” our staff found that author E. Cobham Brewer discusses the substance of this story in the revised edition of his book The Reader’s Handbook of Famous Names in Fiction, Allusions, References, Proverbs, Plots, Stories, and Poems.

According to Brewer, the legend originated in Harry Beaumont’s 1753 book Moralities:

The substance of the tale is as follows: Two knights, approaching each other from opposite directions, came in sight of a trophy shield, one side of which was gold and the other silver. Like the disputants about the chameleon, they could not agree. “What a wonderful gold trophy is that yonder!” said one of the knights. “Gold!” exclaimed the other. “Why, do you think I’ve lost my sight? It is not gold, but silver.” “Tis gold, I maintain”; “Tis silver I insist on.” From words they almost came to blows, when luckily came by a stranger, to whom they referred the dispute, and were told that both were wrong and both were right, seeing one side of it was gold and the other side silver. ²