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.

Blog posts

The Meditation on Curiosity
Why truth? And... — You have an instrumental motive to care about the truth of your beliefs about anything you care about.
Belief in Self-Deception — Deceiving yourself is harder than it seems. What looks like a successively adopted false belief may actually be just a belief in false belief.
The Bottom Line
A Rational Argument
Tarski Statements as Rationalist Exercise by Vladimir Nesov
A Fable of Science and Politics -- characters discover the color of the sky, with political implications.

See also