Actualism arises in response to possibilism, the view that there are, or at least could have been, mere possibilia, i.e., things that are not actual, but which could have been. The possibilist thus draws a distinction between being, in the broadest sense, and actuality, or actual existence. The actualist denies any such distinction. That is, for the actualist, necessarily, everything there is, everything that has being in the broadest sense, is actual. Modern versions of possibilism are typically motivated by concerns about the ground of certain intuitive possibilities, for example, that Wittgenstein (who was childless) could have had a child. Possibilists typically argue that, in order for this to be true, there must be something that could have been Wittgenstein's child and, hence, that grounds the possibility in question — an inference that receives clear formal expression in the principle of quantified modal logic known as the Barcan formula: ◇∃xFx → ∃x◇Fx. Since (the possibilist continues) no actually existing individual could have been Wittgenstein's child (given the plausible view that no one could have had different parents and that no non-human thing could have been been human), Wittgenstein's possible children must be mere possibilia. Actualists typically respond to this reasoning either by postulating actually existing "surrogates" of one sort or another — special properties, for example — to play the grounding role of possibilia or by denying outright the need to ground possibilities like the one in question in the modal properties of possibilia or actualist surrogates thereof. There is broad agreement that possibilism has explanatory power that actualism lacks but the view is widely (but by no means universally) considered philosophically untenable. At the same time, actualism faces several considerable logical challenges. The simplest and most straightforward quantified modal logic — often called SQML — is simply an amalgam of the basic modal propositional system S5 and standard first-order logic. However, the Barcan formula and several other controversial principles turn out to be theorems of SQML and it is by no means obvious whether SQML can be modified in an actualistically acceptable and logically feasible way so as to render these principles invalid.