Truthmaker theory

Truthmaker theory is "the branch of metaphysics that explores the relationships between what is true and what exists". The basic intuition behind truthmaker theory is that truth depends on being. For example, a perceptual experience of a green tree may be said to be true because there actually is a green tree. But if there were no tree there, it would be false. So the experience by itself does not ensure its truth or falsehood, it depends on something else. Expressed more generally, truthmaker theory is the thesis that "the truth of truthbearers depends on the existence of truthmakers". A perceptual experience is the truthbearer in the example above. Various representational entities, like beliefs, thoughts or assertions can act as truthbearers. Truthmaker theorists are divided about what type of entity plays the role of truthmaker; popular candidates include states of affairs and tropes.

Truthmaker maximalism is the thesis that every truth has a truthmaker. An alternative view is truthmaker atomism, the thesis that only atomic sentences have truthmakers. Truthmaker atomism remains true to the basic intuition that truth depends on being by holding that the truth of molecular sentences depends on the truth of atomic sentences, whose truth in turn depends on being.

All non-maximalist positions accept that there are truthmaker gaps: truths without truthmakers. Opponents have tried to disprove truthmaker theory by showing that there are so-called deep truthmaker gaps: truthbearers that not only lack a truthmaker but whose truths do not even depend on being. Various principles governing the truthmaking relation have been proposed in order to make the intuitions about the role and nature of truthmaking explicit. Truthmaker theory is closely related to the correspondence theory of truth, but not identical to it. Truthmaker theory has been applied to various fields in metaphysics, often with the goal of exposing ontological cheaters: theorists who are committed to certain beliefs but do not or cannot account for the existence of a truthmaker for these beliefs.

In Truth-Makers (1984), Kevin Mulligan, Peter Simons and Barry Smith introduced the truth-maker idea as a contribution to the correspondence theory of truth. Logically atomic empirical sentences such as "John kissed Mary" have truthmakers, typically events or tropes corresponding to the main verbs of the sentences in question. Mulligan et al. explore extensions of this idea to sentences of other sorts, but they do not embrace any position of truthmaker maximalism, according to which every truthbearer has a truthmaker.

This maximalist position leads to philosophical difficulties, such as the question of what the truthmaker for an ethical, modal or mathematical truthbearer could be. Someone who is deeply enough committed to truthmakers and who simultaneously doubts that a truthmaker could be found for a certain kind of truthbearer will simply deny that that truthbearer could be true. Those who find the Parmenidean insight sufficiently compelling often take it to be a particularly enlightening metaphysical pursuit to search for truthmakers of these kinds of propositions.

Another difficulty for the claim that every truthbearer has a truthmaker is with negations of existential propositions (or, equivalently, universal propositions). In the example of asking if unicorns exist, proposals include the totality of all things, or some worldly state of affairs such as x1's not being a unicorn, x2's not being a unicorn, ..., and everything's being x1, or x2, or ... (the latter suggestion is due to Richard M. Gale).

David Lewis has proposed a more moderate version of the truthmaker theory on which truthmakers are only required for positive propositions (e.g., there must be a truthmaker for the proposition that there are horses, but not for the equally true proposition that there are no unicorns). What makes a negative proposition p true is the lack of a falsemaker for it, i.e., the lack of a truthmaker for the negation of p. Thus what makes it true that there are no unicorns is the lack of a truthmaker for the proposition that there are unicorns, i.e., the lack of unicorns.

Truthmaker theorists differ as to what entities are the truthmakers of various truthbearers. Some say that the truthmaker of the proposition that Socrates is sitting (assuming Socrates is) is "Socrates' being seated" (whatever exactly that might turn out to be on the correct ontology) and in general the truthmaker of the truthbearer expressed by a sentence s can be denoted by the participial nominalization of s. Others will say that the truthmaker of the proposition that Socrates is sitting is just "Socrates" himself. In any case, the truthmaker is supposed to be something concrete, and on the first view is that whose existence is reported by the truthbearer and on the second view is that which the truthbearer is about.