 A stit model has an additional component: a valuation. A valuation in a frame, it turns out, is a function that assigns to a variable and each index either 1 (truth) or 0 (falsity), where an index is an ordered pair consisting of a history and a moment on that history. The notion of truth or falsity of a formula with respect to an index can now be defined.  If \(V\) is the valuation we have the following basic truth-condition for atomic \(\phi\):