A simple maximality principle

In this paper. following an idea of Christophe Chalons, I propose a new kind of forcing axiom. the Maximality Principle. which asserts that any sentence W holding in some forcing extension V-P and all subsequent extensions V-P*C holds already in V. It follows. in fact. that such sentences must also hold in all forcing extensions of V. In modal terms. therefore. the Maximality Principle is expressed by the scheme (lozenge square phi) double right arrowsquare phi. and is equivalent to the modal theory S5. In this article. I prove that the Maximality Principle is relatively consistent with ZFC. A boldface version of the Maximality Principle. obtained by allowing real parameters to appear in phi. is equiconsistent with the scheme asserting that V-delta < V for an inaccessible cardinal delta, which in turn is equiconsistent with the scheme asserting that ORD is Mahlo. The strongest principle along these lines is square (MP) under tilde. which asserts that (MP) under tilde holds in V and all forcing extensions. From this. it follows that 0(#) exists. that x(#) exists for every set x. that projective truth is invariant by forcing. that Woodin cardinals are consistent and much more. Many open questions remain.