 It is difficult to know how to interpret these principles of plenitude. Quantum mechanics diverges from classical physics by replacing of a deterministic model of the universe with a model based on objective probabilities. According to this probabilistic model, there are numerous ways the universe could have evolved from its initial state, each with a certain probability of occurring that is fixed by the laws of nature. Consider some kind of object, say unicorns, whose existence is not ruled out by the initial conditions plus the laws of nature. Then one can distinguish between a weak and a strong version of the principle of plenitude. According to the weak principle, if there is a small finite probability of unicorns existing then given enough time and space unicorns will exist. According to the strong principle, it follows from the theory of quantum mechanics that if it is possible for unicorns to exist then they do exist. One way in which this latter principle may be cashed out is in the ‘many-worlds’ interpretation of quantum mechanics, according to which reality has a branching structure in which every possible outcome is realized.