How did internal realism differ from metaphysical realism for Hilary Putnam?

For Hilary Putnam, the distinction between metaphysical realism and internal realism centers on the relationship between truth, language, and the world.

Metaphysical Realism
Putnam characterized metaphysical (or "external") realism as the view that we can describe the world as it is, independently of our minds, through a direct "correspondence" between language and mind-independent entities. A core tenet of this view is that truth is "radically nonepistemic," meaning even an ideal scientific theory—one that satisfies all rational and aesthetic criteria—could still be false in reality. Putnam rejected this, arguing it is impossible to explain how language can determinately refer to a "ready-made" world without relying on an untenable, mind-independent perspective.12

Internal Realism
In contrast, Putnam’s internal realism identifies truth with "ideal rational acceptability". Rather than seeking a correspondence to a "thing in itself," this view acknowledges that reality can be described in multiple, equally valid ways. By connecting truth to evidence and grounds for belief, Putnam’s approach avoids the skeptical pitfalls of metaphysical realism while maintaining a commitment to objective truth, which he argued is necessary to avoid falling into relativism.12