Although transcendental arguments have been heavily debated in analytic philosophy, there exists no formalization that could distinguish them from other arguments either employing or seeking to ground necessity—e.g., analytic and mathematical. In this chapter I seek to remedy the lack by using counterfactuals to develop a novel formalization of them that captures their most prominent formal characteristic: the inverted order of inferring to a necessary condition rather than to a consequence of a given premise. Furthermore, unlike the standard way of defining transcendental arguments through certain presupposed non-formal features, the formalization allows for the more desirable route of deriving rather than presupposing these features. In this chapter I will demonstrate this for the most ubiquitous feature of transcendental arguments: that they involve and rely on a reference to mental or cognitive capacities. I will further use the formalization and the reference to cognitive capacities to support Barry Stroud’s (1968) contention that metaphysical or objective use of transcendental arguments must resort to idealism. Contrary to Stroud, I do not take this as a sign of their weakness: I believe it was Kant’s intention to originally develop transcendental arguments as the proper method for transcendentally idealistic metaphysics that has no validity in non-idealistic metaphysics.