In philosophy, reality is the state of things as they actually exist, rather than as they may appear or might be imagined. In a wider definition, reality includes everything that is and has been, whether or not it is observable or comprehensible. A still more broad definition includes everything that has existed, exists, or will exist. Philosophers, mathematicians, and other ancient and modern thinkers, such as Aristotle, Plato, Frege, Wittgenstein, and Russell, have made a distinction between thought corresponding to reality, coherent abstractions, and that which cannot even be rationally thought. By contrast existence is often restricted solely to that which has physical existence or has a direct basis in it in the way that thoughts do in the brain. Reality is often contrasted with what is imaginary, delusional, in the mind, dreams, what is abstract, what is false, or what is fictional. The truth refers to what is real, while falsity refers to what is not. Fictions are considered not real.