ABSTRACT. Philosophies that are labelled as ’idealist’ are disparate, often conflicting with each other, even on what constitutes the nature of idealism. We can find, however, a common idea that finds expression in all of them: that what is possible is in some sense limited to what is conceivable. This, I will suggest, is the fundamental view of idealism. Not only is this precept at the heart of the philosophies of Berkeley, Schopenhauer, and even Kant, but it can also be found in more contemporary approaches within analytic philosophy. Furthermore, the fundamental differences between these doctrines can be illuminated by considering how they each conform to this view: on what notion of conceivability this view is true of them. Kant’s relation to this precept is, perhaps, the most difficult to establish, but this very ambiguity give us scope to state an influential neo-Kantian form of idealism.