Some of the things we say are true, for instance that ‘2+2=4’ and that ‘Oslo is the capital of Norway’; some of the things we say are false, for instance that ‘2+2=5’ and that ‘Norway is the capital of Sweden’; and some of the things we say are neither true nor false, for instance that ‘this room is filled with demons’ and that ‘a convention of gremlins is studying Hegel’s Logic on the planet Venus’. In Objectivism this third type is called arbitrary.

First, I present the usage of the arbitrary in Objectivism, specifically by Ayn Rand, Leonard Peikoff and others. Second, I compare their usage with the ordinary language usage of the term. Third, one might wonder why we need, within philosophy, a concept of the arbitrary. On the face of it, its usage appears to be polemical: through saying that a thesis is arbitrary one justifies rejecting the thesis—at least until some evidence is advanced in favour of the thesis. And on this view, the arbitrary would simply be that which fails the onus of proof principle. However, I will argue that the Objectivist discussion of the arbitrary serves a fundamental role within epistemology and philosophy of language, especially on the importance of evidence. As a result, the concept of the arbitrary justifies the onus of proof principle.

Lastly, I compare the Objectivist usage of the arbitrary with (1) the Epicurean’s rejection of bivalence (the view that every proposition is either true or false); (2) the Polish logician’s (Lésniewski and Łukasiewicz) introduction of a third value, in addition to the true and the false, namely the indifferent or the possible; (3) the notion of a category-mistake, and of type-theory, discussed amongst others by Gilbert Ryle and Peter Strawson (for instance, that ‘the virtue of honesty is green’). I will argue that the Objectivist usage is especially close to Peter Strawson’s view.