Modal fictionalism is a term used in philosophy, and more specifically in the metaphysics of modality, to describe the position that holds that modality can be analysed in terms of a fiction about possible worlds. The theory comes in two versions: strong and timid. Both positions were first exposed by Gideon Rosen, starting from 1990.

Modal fictionalism is a philosophical perspective that centers on the assertion that possible worlds are fictional entities. This perspective seeks to explain our apparent commitment to possible worlds in a manner akin to our engagement with other fictional constructs, such as ideal gases or frictionless surfaces. One of the pioneering works in this field was presented by Rosen in 1990, wherein he and other scholars formulated modal fictionalism as a theory equating talk of possible worlds with discussions about paradigmatically fictional objects, such as Sherlock Holmes. For example, statements like "There is a (non-actual) possible world at which there are blue swans" are understood through an analogy with "There is a brilliant detective at 221b Baker Street," as proposed by Rosen.

Modal fictionalism involves at least a partial account of how paradigmatically fictional claims are to be treated, asserting that these claims are, in a literal and strict sense, false. According to modal fictionalists, there are no merely possible worlds, situations, outcomes, or objects. In strict terms, there is no sculpture created on a particular morning, even though the potential for its creation existed. Similarly, when a coin flip results in heads, there is no outcome in which it lands tails, strictly speaking.

However, within the context of the modal fiction or the fiction of possible worlds, there exists a (merely possible) sculpture that could have been created that morning and an (unactualized) outcome where the coin lands tails. While discussions about merely possible worlds and objects are generally literally false, more elaborate discussions about what is true according to the fiction of possible worlds are considered literally true.

Some proponents of modal fictionalism, such as Hinckfuss (1993), suggest that discussions about possible worlds should be governed by implicit presuppositions known to be false. This approach ensures that statements in the language of possible worlds do not necessitate a belief in their actual existence but rather commit one to more economical propositions, such as "if there were possible worlds of a certain kind, then..." or "given the presupposition that there are possible worlds...". Alternatively, other accounts of how talk about possible worlds functions may be proposed. For instance, Nolt (1986) proposes treating typical "possibilistic discourse" as a form of make-believe, although the specific theory of make-believe is not explicitly defined. Notably, Stephen Yablo (Yablo 1996) employs Walton's theory of make-believe in his modal fictionalism, which he also refers to as figuralism.

One of the primary advantages of adopting a fictional approach to possible worlds is the ability to utilize the language of possible worlds without committing to their literal existence. This approach is particularly appealing when considering merely possible objects, such as blue swans or dragons, which are often characterized by their non-actual existence.

Central to modal fictionalism are biconditionals that establish connections between truths about necessity and possibility and the contents of the modal fiction. These biconditionals, exemplified by schemas like "Possibly P iff according to the fiction of possible worlds, P is true at some possible world" and "Necessarily P iff according to the fiction of possible worlds, P is true at all possible worlds," are crucial for understanding the relationship between modal claims and the modal fiction. While these biconditionals can inter-define necessity and possibility, their precise workings may vary among different modal fictionalists.

In conclusion, modal fictionalism offers a unique perspective on the nature of possible worlds, allowing for the exploration of these concepts while avoiding the ontological commitment to their actual existence. The diversity of approaches within modal fictionalism highlights the rich philosophical discussions and debates surrounding this intriguing viewpoint.