The Litany of Tarski is a template to remind oneself that beliefs should stem from reality, from what actually is, as opposed to what we want, or what would be convenient. For any statement X, the litany takes the form "If X, I desire to believe that X".
Quoting The Meditation on Curiosity:
If the box contains a diamond,
I desire to believe that the box contains a diamond;
If the box does not contain a diamond,
I desire to believe that the box does not contain a diamond;
Let me not become attached to beliefs I may not want.
The name refers to Alfred Tarski, who sought to define what, exactly, "truth" means.
See also: Map and Territory, Litanies & Mantras.