Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.

His idea was that a number statement such as "2 apples + 2 apples = 4 apples" can be taken in two senses. He proposed three worlds: World One being the physical world or physical states; World Two being the world of mind or mental states ideas and perceptions; and World Three being the body of human knowledge expressed in its manifold forms or the products of the second world made manifest in the materials of the first world (i. Hume I felt was perfectly right in pointing out that induction cannot be logically justified.

He is generally regarded as one of the greatest philosophers of science of the 20th century. In 1992 he was awarded the Kyoto Prize in Arts and Philosophy for "symbolising the open spirit of the 20th century" and for his "enormous influence on the formation of the modern intellectual climate". He is also known for his opposition to the classical justificationist account of knowledge which he replaced with critical rationalism "the first non justificational philosophy of criticism in the history of philosophy".